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SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 


This document contains information affecting the national defense of 
the United States within the meaning of the Espionage Act, 50 U. S. C., 
31 and 32, as amended. Its transmission or the revelation of its contents 
in any manner to an unauthorized person is prohibited by law. 

This volume is classified CONFIDENTIAL in accordance with security 
regulations of the War and Navy Departments because certain chapters 
contain material which was CONFIDENTIAL at the date of printing. 
Other chapters may have had a lower classification or none. The reader 
is advised to consult the War and Navy agencies listed on the reverse 
of this page for the current classification of any material. 


Manuscript and illustrations for this volume were prepared 
for publication by the Summary Reports Group of the 
Columbia University Division of War Research under con¬ 
tract OEMsr-1131 with the Office of Scientific Research and 
Development. This volume was printed and bound by the 
Columbia University Press. 

Distribution of the Summary Technical Report of NDRC 
has been made by the War and Navy Departments. Inquiries 
concerning the availability and distribution of the Summary 
Technical Report volumes and microfilmed and other refer¬ 
ence material should be addressed to the War Department 
Library, Room 1A-522, The Pentagon, Washington 25, D. C., 
or to the Office of Naval Research, Navy Department, Atten¬ 
tion : Reports and Documents Section, Washington 25, D. C. 

Copy No. 


This volume, like the seventy others of the Summary Tech¬ 
nical Report of NDRC, has been written, edited, and printed 
under great pressure. Inevitably there are errors which have 
slipped past Division readers and proofreaders. There may 
be errors of fact not known at time of printing. The author 
has not been able to follow through his writing to the final 
page proof. 

Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these reports 
and sent to recipients of the volume. Your help will make 
this book more useful to other readers and will be of great 
value in preparing any revisions. 


SUMMARY TECHNICAL REPORT OF DIVISION 17, NDRC 


VOLUME 4 


COMBAT TRAINING EQUIPMENT 
AND TESTING DEVICES 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 17 
GEORGE R. HARRISON, CHIEF 


WASHINGTON, D. C., 1946 


WAR DEPART; 


V LIBRARY 

WASh . D. c 



WASHINGTON, D. C. 





NATIONAL DEFENSE RESEARCH COMMITTEE 



James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative 1 

Frank B. Jewett Navy Representative- 

Karl T. Compton Commissioner of Patents :i 

Irvin Stewart, Executive Secretary 


1 Army representatives in order of service: 


Maj. Gen. G. V. Strong 
Maj. Gen. R. C. Moore 
Maj. Gen. C. C. Williams 
Brig. Gen. W. A. Wood, Jr. 

Col. E. A. 


Col. L. A. Denson 
Col. P. R. Faymonville 
Brig. Gen. E. A. Regnier 
Col. M. M. Irvine 
Routheau 


2 Navy representatives in order of service: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Furer 
Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 
Commodore H. A. Schade 
3 Commissioners of Patents in order of service: 
Conway P. Coe Casper W. Ooms 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities 
of warfare, together with contract facilities for carrying 
out these projects and programs, and (2) to administer 
the technical and scientific work of the contracts. More 
specifically, NDRC functioned by initiating research 
projects on requests from the Army or the Navy, or on 
requests from an allied government transmitted through 
the Liaison Office of OSRD, or on its own considered 
initiative as a result of the experience of its members. 
Proposals prepared by the Division, Panel, or Committee 
for research contracts for performance of the work in¬ 
volved in such projects were first reviewed by NDRC, 
and if approved, recommended to the Director of OSRD. 
Upon approval of a proposal by the Director, a contract 
permitting maximum flexibility of scientific effort was 
arranged. The business aspects of the contract, including 
such matters as materials, clearances, vouchers, patents, 
priorities, legal matters, and administration of patent 
matters were handled by the Executive Secretary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 

Division A—Armor and Ordnance 
Division B—Bombs, Fuels, Gases, & Chemical Problems 
Division C—Communication and Transportation 
Division D—Detection, Controls, and Instruments 
Division E—Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three 
administrative divisions, panels, or committees were 
created, each with a chief selected on the basis of his 
outstanding work in the particular field. The NDRC 
members then became a reviewing and advisory group 
to the Director of OSRD. The final organization was 
as follows: 


Division 1—Ballistic Research 

Division 2—Effects of Impact and Explosion 

Division 3—Rocket Ordnance 

Division 4—Ordnance Accessories 

Division 5—New Missiles 

Division 6—Sub-Surface Warfare 

Division 7—Fire Control 

Division 8—Explosives 

Division 9—Chemistry 

Division 10—Absorbents and Aerosols 

Division 11—Chemical Engineering 

Division 12—Transportation 

Division 13—Electrical Communication 

Division 14—Radar 

Division 15—Radio Coordination 

Division 16—Optics and Camouflage 

Division 17—Physics 

Division 18—War Metallurgy 

Division 19—Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 




NDRC FOREWORD 


A S EVENTS of the years preceding 1940 revealed 
more and more clearly the seriousness of the 
world situation, many scientists in this country 
came to realize the need of organizing scientific 
research for service in a national emergency. Rec¬ 
ommendations which they made to the White House 
were given careful and sympathetic attention, and 
as a result the National Defense Research Com¬ 
mittee [NDRC] was formed by Executive Order 
of the President in the summer of 1940. The mem¬ 
bers of NDRC, appointed by the President, were 
instructed to supplement the work of the Army and 
the Navy in the development of the instrumental¬ 
ities of war. A year later, upon the establishment 
of the Office of Scientific Research and Development 
[OSRD], NDRC became one of its units. 

The Summary Technical Report of NDRC is a 
conscientious effort on the part of NDRC to sum¬ 
marize and evaluate its work and to present it in 
a useful and permanent form. It comprises some 
seventy volumes broken into groups corresponding 
to the NDRC Divisions, Panels, and Committees. 

The Summary Technical Report of each Division, 
Panel, or Committee is an integral survey of the 
work of that gx'oup. The first volume of each group’s 
report contains a summary of the report, stating 
the problems presented and the philosophy of at¬ 
tacking them, and summarizing the results of the 
research, development, and training activities under¬ 
taken. Some volumes may be “state of the art” trea¬ 
tises covering subjects to which various research 
groups have contributed information. Others may 
contain descriptions of devices developed in the 
laboratories. A master index of all these divisional, 
panel, and committee reports which together consti¬ 
tute the Summary Technical Report of NDRC is 
contained in a separate volume, which also includes 
the index of a microfilm record of pertinent technical 
laboratory reports and reference material. 

Some of the NDRC-sponsored researches which 
had been declassified by the end of 1945 were of 
sufficient popular interest that it was found desirable 
to report them in the form of monographs, such as 
the series on radar by Division 14 and the monograph 
on sampling inspection by the Applied Mathematics 
Panel. Since the material treated in them is not 
duplicated in the Summary Technical Report of 
NDRC, the monographs are an important part of 
the story of these aspects of NDRC research. 


In contrast to the information on radar, which is 
of widespread interest and much of which is released 
to the public, the research on subsurface warfare is 
largely classified and is of general interest to a more 
restricted group. As a consequence, the report of 
Division 6 is found almost entirely in its Summary 
Technical Report, which runs to over twenty volumes. 
The extent of the work of a Division cannot therefore 
be judged solely by the number of volumes devoted 
to it in the Summary Technical Report of NDRC; 
account must be taken of the monographs and avail¬ 
able reports published elsewhere. 

The research work of Division 17 included a wide 
variety of projects, ranging from the detection of 
land mines to the characteristics of the human ear, 
from helium purity indicators to the telemetering of 
strain gauges, from odographs to sound-ranging de¬ 
vices. It is a tribute to the broad knowledge of the 
Division Chiefs—Paul Klopsteg and, later, George R. 
Harrison—and to the versatility of the men who 
worked under them that so diverse a program was 
handled so competently. 

A considerable portion of the work of Division 17 
had to do with the shattering noise of modern war, 
and answers were sought and supplied to such ques¬ 
tions as: How much noise can a human being stand? 
What clues must the human ear have in order to 
understand a spoken message? How much distortion 
can be tolerated? These and other phases of the Divi¬ 
sion’s work are dealt with in the Summary Technical 
Report prepared under the direction of the Division 
Chief and authorized by him for publication. 

The diversity of the Division’s projects made it 
inevitable that its staff should be composed of men 
with many types of scientific training and that the 
Division should draw on contractors with a wide 
range of experience and skills. The studies of noise, 
in particular, meant that the technical staff must 
include physicists, acousticians, and psychologists. 
For the ability and devotion of these men of many 
aptitudes we express our gratitude. 

Vannevar Bush, Director 
Office of Scientific Research and Development 

J. B. Conant, Chairman 
National Defense Research Committee 



v 




FORE WOK I) 


T he applied physics division of the Office of 
Scientific Research and Development 
[OSRD] was organized late in 1942 under the 
chairmanship of Dr. Paul E. Klopsteg, who was 
responsible for the work of the Division until 
shortly before the completion of its work when 
other duties required his full attention. Most of 
the projects which had been initiated by the 
Instruments Section of the National Defense 
Research Committee [NDRC] during 1940 and 
1941 and which were not concerned with optics 
were turned over to the Applied Physics Divi¬ 
sion on its inauguration. Dr. Klopsteg as Chief 
and Dr. E. A. Eckhardt as Deputy Chief went 
with them from the Instruments Section to the 
new Division. 

The Summary Technical Report which is pre¬ 
sented in these volumes thus covers the accom¬ 
plishments of projects set up by both Section 
D-3 and Division 17. The work of the Division 
covered a very wide range of fields. The term 
“applied physics” served in lieu of a more de¬ 
scriptive name for a Division which was in fact 
the one to which was assigned any scientific 


problem which did not properly come under one 
of the other divisions of NDRC. 

Actually the Division was an association of 
three Sections having rather dissimilar respon¬ 
sibilities and fields of activity. In setting up 
these Sections it was necessary to group the 
projects already under way into a small number 
of coherent categories, and those chosen were 
Sound, Electricity, and General Instrumenta¬ 
tion. The work of the Division consisted entirely 
of the integrated efforts of these three Sections, 
whose membership will be found listed on a suc¬ 
ceeding page. 

For more detailed reports on the technical 
work of the Division than are contained here¬ 
with the detailed contractors’ reports of Divi¬ 
sion 17 should be consulted, and appropriate 
reference to these have been made throughout 
the present volumes. The results obtained are 
also presented in less technical form in that 
volume of the history of OSRD entitled Optics 
and Applied Physics in World War II. 

George R. Harrison 
Chief, Division 17 



vii 






PREFACE 


T he research and development program of 
Division 17 of the National Defense Research 
Committee [NDRC] was concerned with those prob¬ 
lems in physics not specifically covered in other 
Divisions of NDRC. As the result, the Division fell 
heir to a myriad of miscellaneous problems of a 
physical nature which, themselves, were not often 
interrelated. It would have been exceedingly difficult, 
if not impossible, for Division 17 to set up within 
itself a sufficient number of Sections to deal spe¬ 
cifically with all the various classes of problems which 
fell under its jurisdiction. Therefore, the projects 
of the Division were assigned to one of three Sections 
—Section 17.1, Instruments; Section 17.2, Electrical 
Equipment; and Section 17.3, Acoustics—whose 
broad titles permitted a general, even if somewhat 
loose, classification. It was not always easy to decide, 
at times, under which of these three broad categories 
a given project should be placed. In these cases, 
considerations such as immediate convenience and 
availability of experienced personnel were often the 
determining factors. 

The Summary Technical Report describing the 
activities of Division 17 is presented in four volumes. 
In an attempt to achieve a little greater uniformity 
of subject matter, the projects were organized within 
the various volumes without regard to their Section 
classification. Consequently, there is, on the whole, 
little relationship between volume and Section num¬ 
ber. Because of the varied problems dealt with in 
the Division’s program, very little continuity is to 
be found from chapter to chapter in any volume. 
Each chapter attempts to summarize independently 
the results of a particular project. 

Since there were a large number of diversified 
projects in Division 17, it was obviously impossible 
to do justice to each, even in summary. It is not 
intended that the importance of any project described 
herein should be judged by the amount of page space 
allotted to it. Naturally, certain problems involved 
more research and development than others before 
they could be brought to a successful conclusion. In 
many cases, this is reflected in the Summary Tech¬ 


nical Report. On the other hand, the presentation 
of the projects may mirror the enthusiasm (or lack 
of it) of the individual author at the time of writing. 
Therefore, the reader who desires more than a broad 
panorama of the Division’s activities is referred to 
the Microfilm Index for more complete details. 

This is the fourth and final volume of the Summary 
Technical Report of Division 17. The devices and 
techniques discussed herein are for home-front in¬ 
strumentation rather than for battle-front instru¬ 
mentation. That is, this volume deals with devices 
and techniques which would affect the battle front 
indirectly, through facilitating production and/or 
perfection of materiel or through improving the 
training and preparations of personnel for battle. 

Each chapter in this volume discusses either a 
single Division 17 project or a related group of 
projects. As shown by the by-lines in the Contents 
and in the chapter headings, a number of authors 
have assisted in preparing this book. This oppor¬ 
tunity is taken to express sincere appreciation for 
such assistance. It should be borne in mind that 
although every reasonable effort has been made to 
keep this book free from error, the authors have 
ordinarily not been setting forth their own personal 
experiences or results of personal research. The 
chapters are derived from careful and conscientious 
study of contractors’ reports and associated pub¬ 
lications. Neither the authors nor the Office of 
Scientific Research and Development [OSRD] are 
accountable for the correctness of the facts which 
form the basis for this volume. 

This opportunity is taken to express appreciation 
for other assistance in connection with this volume: 
the research done for the OSRD under contract and 
reported herein; the time and effort expended by 
personnel of Division 17 and its Sections in reading 
and criticizing manuscripts; and the care exercised 
by the Summary Reports Group in publishing 
the material. 

F. L. Yost 
Editor 




IX 




































\ 




















































CONTENTS 


CHAPTER PAGE 

1 Telemetering of Strain Gauges and Instruments by 

George E. Beggs, Jr . 1 

2 The Acoustic Firing Error Indicator by J. W. M. Du Mond 

and E. R. Coh en .38 

3 Magnetic Recording Research by George E. Beggs, Jr. . 88 

4 Oscillographs by George E. Beggs, Jr .108 

5 High-Voltage X-Ray Radiography by John A. Hornbeck 135 

6 Bomb Instrumentation by F. L. Yost .146 

7 Deflection-Time Measuring Devices by F. L. Yost . . 155 

8 Measurement of Wall Thicknesses of Hollow Steel Pro¬ 
peller Blades by F. L. Yost .162 

9 Development and Applications of Electronic Counter Cir¬ 
cuits by George E. Beggs, Jr., and F. L. Yost . . . 167 

10 Development of Methods for Detecting Defective Rotat¬ 
ing Bands on Projectiles by Clark Goodman .... 177 

11 Helium-Purity Indicators by F. L. Yost .183 

12 Battle Noise Reproduction for Training and Screening 

Battle Personnel.189 

13 Sound Spectrum of Ordnance Equipment and Battle 

Noises.191 

14 D3 Projects Reported by Division 17 by J. S. Coleman . 193 

Glossary.201 

Bibliography.205 

OSRD Appointees.212 

Contract Numbers.214 

Survey Project Numbers.220 

Index.221 


TTflTrTTfFNTTM/ 

















































» 





















































Chapter 1 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS a 

By George E. Beggs, Jr. b 


ABSTRACT 

His report describes the results of various 
projects conducted under the supervision of 
the National Defense Research Committee in the 
field of telemetering of strain gauges and other 
instruments from aircraft to ground by a radio 
link, to allow remote-control testing of high¬ 
speed aircraft. There is included a rather gen¬ 
eral discussion of various systems of multiplex¬ 
ing to permit the handling of numerous channels 
of information by the radio link. This report 
presents a general survey of the telemetering 
field as of September 1945, including develop¬ 
ments made by groups under Service or NDRC 
supervision. 

A brief description of the application of cer¬ 
tain telemetering principles to transmission of 
data from guided missiles is also included. 

12 INTRODUCTION 

Development of small, high-speed aircraft, 
approaching sonic speeds under certain condi¬ 
tions, has made flight testing of experimental 
and production prototypes increasingly difficult 
by methods in vogue at the beginning of the war. 
The phenomena of compressibility, flutter, and 
other instabilities evident at high speed de¬ 
manded very accurate means for measurement 
and analysis of dynamic data concerned with 
stresses and accelerations within the aircraft. 
Furthermore, to allow reasonably complete an¬ 
alysis of the structure, many items of informa¬ 
tion were desired simultaneously. Such observa¬ 
tion and recording could not be done by a test 
pilot, even if he were riding only as a passenger, 
without part of his attention necessarily being 
devoted to the flying. 

Originally, methods were employed in which 
data of the type noted above were recorded on 
multi-channel oscillographs mounted in the air- 

a AC-40, NA-133, NA-134, NA-152, NA-242. 

b Technical Aide, Section 17.1-17.2, NDRC. 


craft, the pilot being present only to put the 
plane through appropriate maneuvers. Numer¬ 
ous test flights of this type were made, and valu¬ 
able data were obtained. However, testing under 
conditions of greatest severity was seldom pos¬ 
sible, due to the human element. In addition, 
structural failures causing loss of the aircraft 
often resulted in loss of the records. 

It became apparent that complete tests could 
be obtained only by remote radio control of a 
pilotless aircraft, the data being transmitted by 
radio to a ground recording station. Television 
apparatus was used in some cases, but a need for 
intelligence data to be transmitted at higher fre¬ 
quency emphasized the necessity for equipment 
specifically developed to transmit strain-gauge, 
accelerometer, and pressure data. The process of 
transmitting multi-channel data is known as 
telemetering and, in the case of aircraft tele¬ 
metering, involves the use of a means of multi¬ 
plexing numerous intelligence channels on a 
single radio link. Multiple radio links are un¬ 
desirable, since little space is available for mul¬ 
tiple-antenna installations and duplication of 
numerous radio circuits is uneconomical with 
respect to materials, space, and power. 

In addition to the requirements for telemeter¬ 
ing apparatus for aircraft, there arose later in 
the program a need for similar, more compact 
apparatus for the development and testing of 
guided missiles. In this case the need was even 
more urgent, since no data could be obtained by 
the use of a pilot even in early test stages. The 
basic principles of aircraft telemetering proved 
applicable to the development of this more spe¬ 
cialized equipment. 

13 MILITARY REQUIREMENTS 

Basically, military requirements for tele¬ 
metering are constantly in a state of flux, since 
available test data from any given system in¬ 
variably indicate the need for more channels, 



ORDER SEC 


Degraded unclassified „ 
:C ASSAY BY TAG Wife, 0 5 ^ x ,j 








TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


greater accuracy, higher frequency response, 
less weight, size, power consumption, and other 
improvements. 

The latest phase of telemetering completed 
by NDRC demanded the following charac¬ 
teristics : 

1. A minimum of 14 channels of information. 

2. Each channel to respond to intelligence 
variations of 0 to 200 c. 

3. Intermodulation and random inaccuracies 
to be as low as possible, preferably less than 2 
per cent. 

4. Transmission by a single radio link. 

5. Ground recording by a standard multi¬ 
channel recording oscillograph. 

6. Adaptability to signals from wire strain 
gauges, accelerometers, and other similar 
devices. 

7. Small, lightweight, reliable, low power- 
consumption airborne apparatus operable from 
24- to 30-v d-c supply. 

8. Range of 30 miles or more, regardless of 
aircraft attitude. 

Various groups attempted several approaches 
to the problem, one of which led to a successful 
solution. 

^ SUMMARY 

Early work in the field of telemetering was in¬ 
itiated because of Army requests for apparatus 
which would transmit data from instruments in 
guided missiles (i.e., pilotless aircraft or con¬ 
trolled bombs) to a control location, so that the 
operator could maneuver and guide the missile 
in offensive operations. As the development pro¬ 
gressed along various lines of instrument tele¬ 
metering, new interest arose in the possibility of 
applying the transmission of data in this manner 
to the study of new types of aircraft under test, 
thus somewhat changing the emphasis of the 
program. 

Early in 1943, testing of high-speed aircraft 
introduced the necessity for transmission of 
data derived from resistance strain gauges and 
instruments of a similar nature, with a relatively 
high degree of accuracy and over a large number 
of channels. The final equipment developed has 
proved sufficiently versatile to transmit readings 
derived from standard, or from somewhat mod¬ 


ified aircraft instruments and from strain 
gauges, allowing application of the apparatus to 
numerous and diverse problems of testing and 
operation. 

In the field of instrument telemetering, three 
contracts were initiated under NDRC. The con¬ 
tract with the National Broadcasting Company 
[NBC] 1 ' 3 involved the development of a rela¬ 
tively narrow-band television system of low defi¬ 
nition for reproducing instrument readings. 
These readings made by the television camera 
in the aircraft were to register at a ground lo¬ 
cation with sufficient clarity to obtain data with 
the desired degree of accuracy. This system was 
intended to replace the Block I e television set 
which was comparatively bulky and required 
considerable band width in the radio-frequency 
spectrum, although it gave good quality recep¬ 
tion. The NBC program utilized the new 2-in. 
Orthicon camera tube as a basis for a new low- 
quality system of 10 frames per second, 200 
lines. The development of this system was com¬ 
pleted early in 1943 and flight tests of the ap¬ 
paratus were made with the assistance of the 
Aircraft Radio Laboratory at Wright Field. 
Although the flight tests proved satisfactory, 
it was found that new developments in the field 
of high-definition television, specifically Block 
III C equipment, superseded the low-definition 
type. Since band-width requirements no longer 
appeared to be critical and the bulk and weight 
of the Block III system were comparable to the 
NBC-NDRC developments, this system was 
chosen. Furthermore, emphasis at this time 
shifted to the transmission of strain-gauge data 
which could be handled in some degree more 
satisfactorily by other means. 

The Hazeltine Electronics Corporation 410 
made an approach to the instrument-telemeter¬ 
ing problem by utilizing a system of line scan¬ 
ning essentially in one dimension, rather than 
the two-dimensional used in television. This al¬ 
lowed reduction of the required band width by 
transmitting only pertinent data on the position 
of instrument pointers and the scale zeros, with¬ 
out transmitting unnecessary data on the gen¬ 
eral appearance of the panel. The zero and a mov¬ 
ing pointer on each instrument were equipped 

' Army-Navy identification of airborne television appara¬ 
tus developed by RCA, Camden, N. J. 






SUMMARY 


3 


with magnetic strips. The pointer position was 
determined by scanning the instrument dial with 
a pickup coil of high permeability, mechanically 
driven in a circle concentric with the dial. The 
pulses produced in the pickup by the zero and 
pointer magnets were transmitted in time se¬ 
quence over a standard communication link and 
separated at the receiving end. Following sepa¬ 
ration, the items were applied to electronically 
scanned cathode-ray tubes. The pulses indicating 
pointer position were used to intensify the scan¬ 
ning trace at the appropriate times so that the 
face of the cathode-ray tube presented two dots 
angularly separated by the same amount as 
the original zero and pointer. This is essentially 
a mechanical commutation system. It was 
flight-tested at Wright Field and performed sat¬ 
isfactorily but its usefulness was limited by its 
inability to transmit rapid variations in instru¬ 
ment readings, and by its size and mechanical 
complexity at the transmitting end. Thus, with a 
shift in interest from telemetering aircraft in¬ 
struments to telemetering strain-gauge indica¬ 
tions and with the development of the Block III 
television apparatus under Service supervision, 
no applications of this development ensued. 

The Rudolph Wurlitzer Company, 11 ' 12 under 
NDRC contract, approached the problem of in¬ 
strument telemetering by the method of subcar¬ 
rier pulse modulation, the subcarriers being 
combined and transmitted over a single radio 
link to be separated at the receiving end by band¬ 
pass filters. The pulse length transmitted for any 
instrument represented the indication of the in¬ 
strument in a linear relation. This system was 
basically sound and proved itself relatively satis¬ 
factory in flight tests, but the adaptation of 
standard instruments to produce variable pulse 
lengths required complex electromechanical 
transducers adapted from Magnesyn-Autosyn 
systems developed by Bendix. (See Figures 16, 
17, 18, 19, and 20.) Furthermore, the pulsing 
rate of the system was too low to allow trans¬ 
mission of other than slow variations of the in¬ 
struments. The change in emphasis to the strain- 
gauge program required the adaptation of this 
system to strain-gauge transmission which was 
quite difficult. 

With the advent of an intensive development 
of high-speed fighter aircraft there was increas¬ 


ing need for a great number of channels to be 
transmitted from strain gauges of the wire- 
resistance type to observe flutter and various 
peak-load conditions. The background knowl¬ 
edge gained from the instrument-telemetering 
program showed that there were difficulties en¬ 
countered from intermodulation between some 
channels, that the intelligence response was not 
of sufficiently high frequency, and that the num¬ 
ber of channels available (with the exception of 
television, five at most) was inadequate. It ap¬ 
peared desirable therefore to make a general 
survey of methods for telemetering strain-gauge 
information, as well as instrument indications, 
and to set up a comprehensive program to study 
these basic problems and to develop apparatus 
capable of satisfying the aircraft test program. 

Accordingly, in April 1943, two sets of speci¬ 
fications were drawn up covering approaches to 
the strain-gauge telemetering development by 
subcarrier transmission of intelligence and by 
time-division multiplexing (i.e., commutation). 
In both cases the initial requirements were sub¬ 
stantially the same: the intelligence frequency 
to be transmitted should be of the order of 100 c 
to 200 c, and a minimum of 14 (preferably 20) 
channels of information should be available for 
transmission over a single radio link with ac¬ 
curacies within 2 to 5 per cent on any channel. 
In addition, it was requested that the basic dif¬ 
ferences between the two types of multiplexing 
be studied to see under what conditions a partic¬ 
ular system might have advantages. 

Three new NDRC contract arrangements were 
established to study development of subcarrier 
and time-division multiplexed telemetering: one 
with the Wurlitzer Company, 13 one with C. G. 
Conn, Ltd., 14 ’ 15 and one with Princeton Univer¬ 
sity. 16 * 23 As a result of preliminary work under 
these contracts, the following facts became ap¬ 
parent, making cessation of work on subcarriers 
by Wurlitzer and Conn appear desirable. 

There are essentially two methods of tele¬ 
metering : 

1. Subcarriers (continuous system). 

2. Commutation (intermittent scanning). 

A general discussion of both methods is pre¬ 
sented. The following is a comparison of the re¬ 
quirements on the radio link and associated 
circuits. 



4 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


Subcarriers (1) A high degree of linearity of 
radio link and associated circuits is required to 
avoid cross talk. For example if x per cent of 
third harmonic distortion is present, 6x per cent 
cross modulation appears. The situation is worse 
for higher harmonics. This makes choice of 
center frequencies difficult. 

2. There is no stringent requirement as to fre¬ 
quency response as long as it does not vary ap¬ 
preciably over the pass band of any channel. 

Commutation. (1) Linearity of radio link and 
associated circuits need be of sufficient degree 
for reduction of data. If calibration curves are 
used, linearity is relatively nonessential. 

2. In order to avoid cross talk, the frequency 
response of the radio link must reproduce the 
composite signal adequately. This is discussed in 
the body of the report. 

15 NOISE CONSIDERATIONS 

Theoretical considerations have shown that 
commutation has advantages over subcarriers 
with respect to both fluctuation and impulse 
noise. 21 This advantage is more pronounced as 
the number of channels is increased. The reason 
is that in commutation each channel occupies 
the whole modulation range, whereas this range 
must be divided among the subcarriers requiring 
considerable separation between them in the 
frequency selector. 

The choice between subcarriers and commuta¬ 
tion will depend upon the number of channels, 
required frequency response of each channel, re¬ 
quired intelligence-to-noise ratio, nature of radio 
link at hand, etc. If a number of slowly varying 
channels are to be handled, it appears that com¬ 
mutation, mechanical or otherwise, has advan¬ 
tages in equipment and performance. If merely 
a few rapidly varying channels (frequency res¬ 
ponse to 150 c, for example) are required, it is 
difficult to estimate from theoretical considera¬ 
tions alone which system would be better. If 20 
or more channels are needed, it appears that 
commutation has advantages as to intelligence- 
to-noise ratio and as to linearity requirements 
of the radio link and associated circuits. 

It is suggested that in the case where reactance 
gauges, such as reactance accelerometers, can 
be used, some improvement in signal-to-noise 


ratio can be obtained if sufficient band width is 
available. This is accomplished by using the re¬ 
actance gauge to modulate the frequency of the 
subcarrier oscillator and by using a limiter and 
a discriminator after the frequency selector at 
the receiving end, i.e., subcarrier frequency mod¬ 
ulation. 

Both general methods of telemetering have 
been tested under diverse requirements and con¬ 
ditions by various organizations. A brief sum¬ 
mary of the work of these organizations is in¬ 
cluded in this report. The general consideration 
of subcarriers vs commutation might be sum¬ 
marized as follows: Multiplexing by subcarriers 
demands good linearity of the transmitting me¬ 
dium from an amplitude standpoint, to prevent 
intermodulation between the various subcar¬ 
riers, but does not demand good frequency re¬ 
sponse, since the relative amplitudes of the 
subcarriers are unimportant. However, commu¬ 
tation demands good frequency response to avoid 
distortion of the square pulses derived from the 
switching circuits and corresponding cross talk 
between channels, but amplitude nonlinearity 
will not cause intermodulation between channels 
although it will cause nonlinearity of any given 
channel. The latter can be removed by calibra¬ 
tion of each channel. In general, a radio link can 
be made relatively satisfactory from a fre¬ 
quency-response standpoint, but is difficult to 
make linear to a wide range of amplitudes. Ac¬ 
cordingly, commutation lends itself to multiplex¬ 
ing via radio link, in numerous cases producing 
more satisfactory results than subcarrier multi¬ 
plexing. 


16 DESCRIPTION AND TECHNICAL 
INFORMATION 

General Discussion 

For the flight testing of aircraft it is often de¬ 
sirable to transmit data, such as instrument 
readings, strain-gauge and accelerometer indi¬ 
cations, etc., via a radio link to a ground station 
where the data can be recorded. This process is 
generally referred to as telemetering. In this re¬ 
port each individual sequence of data (e.g., in¬ 
formation from a particular strain gauge) is 
called a channel. It is generally desired to tele- 






DESCRIPTION AND TECHNICAL INFORMATION 


5 


meter a number of channels. On account of 
power, space, weight, antenna, and other con¬ 
siderations, it is usually undesirable to provide a 
separate radio link for each channel. It is, there¬ 
fore, necessary to provide some means of trans¬ 
mitting a number of channels over the same 
radio link. Figure 1 is a generalized block dia¬ 
gram of a telemetering system for transmitting 
and recording n (see Section 1.7) channels of in- 



Figure 1. Block diagram of basic telemetering system. 


formation. The device which reduces the n chan¬ 
nels to one electric channel to modulate the radio 
transmitter may be a subcarrier system, a com¬ 
mutator, either electric or mechanical, a televi¬ 
sion-scanning device, or a combination of all 
three. The corresponding receiving device which 
records all channels will be a frequency selector 
and recorder, if subcarriers are used; a synchro¬ 
nized commutator and recorder (for slow com¬ 
mutators a recorder is sometimes sufficient), if 
commutation is used; a television screen and 
movie camera, if television is used; or a combina¬ 
tion of all three. 

There are essentially two methods by which 
telemetering can be performed : these two meth¬ 
ods are the subcarrier systems and the commu¬ 
tation systems. 


Subcarrier Systems 

A separate subcarrier frequency is provided 
for each channel. This is usually in the range 
from 1 to 50 kc, and is called a subcarrier be¬ 
cause the range is much lower in frequency than 
the radio carrier. This subcarrier is modulated, 
either in amplitude or in frequency, with the in¬ 
formation of the channel. Most systems use am¬ 
plitude modulation. The subcarriers are then lin¬ 
early superimposed (mixed) and transmitted by 
radio to the receiving station. There the various 
subcarriers are selected, amplified, detected, and 
fed to the recording instrument. The frequency 
selection can be accomplished in several ways. 

Ordinary Subcarrier System. A separate 
band-pass filter is used for each subcarrier, the 
frequency of the subcarrier being adjusted to 
the center of the pass band. The band-pass filters 



Figure 2. Block diagram of subcarrier telemetering 
system for n channels using n band-pass filters. 


are usually designed to operate in parallel. Fig¬ 
ure 2 is a block diagram of this method. 

Heterodyne Subcarrier System. At the re¬ 
ceiver, each channel is provided with a local os¬ 
cillator which beats against its corresponding 
subcarrier to give a certain frequency which is 
approximately in the center of a band-pass filter. 
Thus n identical band-pass filters can be used in¬ 
stead of the n different filters required in the 


















































6 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


ordinary method. The output of each filter is then 
amplified, detected, and fed to its recording in¬ 
strument. Figure 3 is a block diagram of the 
receiving part of this system. The transmitting 
part is identical with Figure 2. 

The Wattmeter Principle. The conventional 
wattmeter has a current coil and a voltage coil. 


RADIO 

RECEIVER 


LOCAL 
OSCILLATOR 
NO. 1 


MIXER 

NQ.1 


FILTER 

NQ.1 


. RECORDER 
) ELEMENT 
NO. 1 




LOCAL 
OSCILLATOR 
NO. 2 



MIXER 

NO. 2 





FILTER 
NO. 2 


AMPLIFIER 
DETECTOR 
NO. 2 


-O RECORDER 
-o ELEMENT 
NO. 2 


PLUS n-2 ADDITIONAL CHANNELS 


Figure 3. Block diagram of receiving portion of hetero¬ 
dyne subcarrier system. 


If the period of the meter is long compared to the 
periods of the current and voltage, the reading 
of the meter is proportional to the time average 
of the product of the current times the voltage, 
i.e., proportional to the time average of the prod¬ 
uct of the currents through the coils. For the pur¬ 
pose of frequency selection, one wattmeter would 
be provided for each channel. In the case of a 
single channel, e.g., the mth one, the composite 
signal from the radio receiver would be applied 
to one coil of the wattmeter, and a signal from a 
local oscillator of constant amplitude and of the 
same frequency as the mth. subcarrier would be 
applied to the other coil. If the period of the 
wattmeter is long compared to that of the sub¬ 
carrier but short compared to that of the modu¬ 
lation, the reading of the meter, which is pro¬ 
portional to the time average of the product of 
the two signals applied to the coils, would be 
proportional to the amplitude of the mth sub¬ 
carrier times the cosine of the phase angle be¬ 
tween the mth subcarrier and the mth local 
signal. All other subcarrier frequencies average 
out. Figure 4 is a schematic diagram of the watt¬ 
meter frequency selector. The transmitting cir¬ 
cuit is the same as that of Figure 2. Care must 
be taken to keep the amplitude of the local oscil¬ 
lator constant and the phase angle between cor¬ 


responding subcarriers and local oscillators the 
same. This means that the corresponding fre¬ 
quencies must be exactly equal. 

In practice, the wattmeter, a square law de¬ 
vice, is not used; instead, electric circuits, which 
achieve the same results, are employed. These 
circuits make use of a nonlinear device which 
gives the product of the local generator signal 
and the composite signal from the radio receiver. 
By means of balanced circuits, filters, etc., it is 
possible to select the desired frequency. 

Frequency-Modulated Subcarrier System. 
There is at least one system in use in which the 
subcarriers are frequency-modulated. Each sub¬ 
carrier oscillator has a variable reactance in the 
tank circuit which modulates the natural fre¬ 
quency of the oscillator. This reactance is con¬ 
trolled by an accelerometer, strain gauge, or 
other device. It is convenient to make the fre¬ 
quencies of the subcarrier oscillators rather high 
(order of 100 kc) so that the frequency deviation 
may be several per cent. 21b 

In general, the use of frequency-modulated 
subcarriers with the resistance strain gauges re¬ 
quires amplification to a relatively high level, 



Figure 4. Block diagram of wattmeter method of fre¬ 
quency selection. 


plus frequency modulation by means of a reac¬ 
tance tube or phase modulator. However, with 
these types of modulation it is difficult to obtain 
a high deviation ratio. Reactance-type strain 
bridges can be used conveniently only in special 
cases because of their size, accurate machining 


rpi T i T FIP FI f ^TtTr 

















































DESCRIPTION AND TECHNICAL INFORMATION 


required, dependence upon vibration, etc. Also a 
considerably larger band width is required to 
handle the frequency-modulation side frequen¬ 
cies. 

The receiver consists of a frequency selector 
and a limiter and discriminator for each channel. 
The results are good only if the signal strength 
is great enough to saturate the limiters. 

Operation and Limitations of Subcarrier Sys¬ 
tems. For specific purposes the subcarriers may 
be modulated in many ways. One common ex¬ 
ample is the application to a strain-gauge bridge 
in a particular channel. The input of the bridge 
is usually the subcarrier frequency of that 
channel and the output of the bridge is then the 
subcarrier frequency amplitude-modulated in 
proportion to the strains. Another example is 
an accelerometer the amplitude of which modu¬ 
lates the subcarrier in proportion to the ac¬ 
celeration. 

The following requirements must be met for 
the application of the subcarrier systems: 

1. Linearity of radio link. All circuits from the 
transmitter input to the mixer to the filter input 
at the receiver must be linear to avoid cross mod¬ 
ulation. Let the subcarrier frequencies be f lt / 2 , 
f 3 Let the response of the link from the 

input of the mixer, v if and its output to the 
receiver frequency selector, v 0 , (assuming the 
frequency selector to be linear) be related by the 
power series 

v 0 = diVi + a 2 Vi 2 + a 3 Vi S -\ - fa//, (l) 


If there is complete linearity, all a’s except a x 
are zero. If the link is nonlinear, various powers 
of Vi appear with coefficients which depend upon 
the nature of the nonlinearity. The effect of non¬ 
linearity is to introduce various modulation 
products resulting in the generation of new 
frequencies which are the sums of various linear 
combinations of the subcarrier frequencies. It 
can be shown that the new frequencies intro¬ 
duced by each term of equation (1) are given 
by all possible combinations of the sum 

± m 2 / 2 ± m 3 / 3 ± ... ± m n f n (2) 
in which the m/s are positive integers including 
zero, such that 

2 mj = p (3) 

j=i 

in which /? is the power of v t in the particular 
term of the power series of equation (1). Table 1 
gives the frequencies which are generated by 
nonlinearity for values of p up to 4. For this 
table only unmodulated subcarriers are consid¬ 
ered and the amplitude of each is set to unity. 
The a/s are the coefficients in equation (1). 

Take for example a certain subcarrier system 
having frequencies 10,833 to 43,333 in steps of 
2,500 c. 

Table 1 gives the cross-modulation frequencies 
generated by the term in the power series of 
equation (1) whose exponent is /?. Consider the 
effect of the nonlinearity which brings in only 
the first and second powers of v it Table 1 shows 


Table 1 


Amplitude 
relative to 
/3th harmonic 

P 

Amplitude 

Cross-modulation frequencies 

1 

1 


a f 

/!,/*,/»•••/» 

1 

2 ] 

r 

1/2 a,2 

2 / 4 , 2 / 3 , 273 , 2 / 4 , • • • 2 f n 

2 

1 

L 

a 2 

fl =fc/ 2 ,/l ±/j, ’ ' ’ fl dcfnifi ±A>A ±/l, • • */ 2 ±/nj etc. 

9 

1 

r 

9/4a 3 

A,AA, ' • •/» 

1 

3 J 


l/4os 

3/i,3/ 2 ,3/ 3 , • • • 3 fn 

3 

1 


3/4a 3 

2 /, ±/ 2 , 2 /i ±/*, • • • 2 /i ±/„; 2/ 2 ±/i, 2/ 2 ±A,2/ : ±/„; etc. 

6 

1 


3/2a 3 

fl ±/ 2 ifc/s, fl ±/ 2 =fcA> ' ‘ * A ±/ 2 ±/n,'/l ±/s ±/ 4 ; etc. 

16 


r 

2a, 

2 / 1 ,2/ 2 , • • • 2 fn 

1 



1/8 a. 

4/l,4/ 2 , • • • 4 fn 

36 



9/2 a, 

fi ±/ 2 , • ■ 'fi ±fn>fz =tjTa, • • */ 2 ±/»; etc. 

6 

4 < 


3/4 a, 

2 fl ± 2ft, ■ • • 2 /, ± 2 f n \ 2ft ± 2 / 3 , • • • 2 if, ± 2/„; etc. 

4 



1/204 

3 fi ±ft, ■ ■ ■ 3 fi ±/»; 3 / 2 ±/i, • ■ ■ 3/, ±/„; etc. 

12 



3/2o 4 

2 fi ±L ±/«, ■ • • 2/i ±/ 2 ±/„; 2A ±/ 3 ±/«, • • • 2/i ±/ 3 ±/„;etc. 

24 



3a4 

A ±/ 2 ±A ± A, • • • /i ± A ±A ±/»>' etc. 












8 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


that there are cross-modulation frequencies 
which are double all subcarrier frequencies, giv¬ 
ing 21,666; 26,666; 31,666; etc. These frequen¬ 
cies lie not closer than 833 c from the center fre¬ 
quencies of the subcarriers. In this particular 
system the frequency selector is down about 20 
db for 800 c off center frequency. Unless a 2 in 
the power expansion of equation (1) is large, 
these double frequencies will consequently not 
cause much trouble. There are also cross-modu¬ 
lation frequencies representing all sum and dif¬ 
ference frequencies, taken two at a time. This 
gives 10,833 +13,333 = 24,166; 10,833 + 15,833 
= 26,666; etc. Inspection shows that these also 
are at least 833 c off center frequency. 

Now consider /? = 3. There are cross-modula¬ 
tion frequencies which are three times the sub¬ 
carrier frequencies, giving 32,499; 39,999; etc. 
These frequencies are at least 834 c off center 
frequency. Also there are all frequencies plus 
or minus twice all other frequencies taken two 
at a time, giving 33,333 — 2 X 10,833 = 11,667; 
35,833 — 2 X 10,833 = 14,167, etc. These are at 
least 834 c off center frequency. But there are also 
all sums and differences, taken three at a time 
giving, for example, 10,833 + 15,833 — 13,333 
= 13,333; 13,333 + 15,833 - 10,833 = 18,333. 
These are exactly on the center frequencies of 
the various subcarriers. Thus, if the subcarriers 
are modulated, it is possible for the modulation 
to cross from one channel into another without 
being attenuated by the frequency selector. 
Furthermore, Table 1 shows that if there is n 
per cent third harmonic distortion, there results 
from the term 6n per cent cross modulation 
without attenuation. For higher values of p 
there are more complicated linear combinations 
of the various frequencies which could cause 
cross modulation in many ways. 

From Table 1 it can be seen that, for a given 
value of p, the modulation sums have larger 
amplitudes than the harmonics. It is therefore 
generally more important to arrange the center 
frequencies of the filters to exclude these modu¬ 
lation sums than to arrange them to exclude 
merely the harmonics. As larger numbers of 
subcarriers are used, this becomes more difficult, 
so that accurate linearity of the radio link, etc., 
becomes imperative. 

2. Modulation of radio link. Figure 5 gives the 


output voltage versus input voltage for a typical 
frequency-modulation radio link. It is reasonably 
linear over a finite input voltage range. In order 
to avoid operation in a badly nonlinear portion, 
the input voltage must never exceed a certain 
value. If the link is to be used to transmit sub- 



Figure 5. Output voltage versus input voltage for 
typical f-m radio link. 

carriers without the possibility of cross modula¬ 
tion, the sum of the instantaneous voltages of 
all the subcarriers must never exceed this defin¬ 
ite value. Let the linear input voltage amplitude 
range be V r . If each subcarrier is not to be modu¬ 
lated more than 100 per cent, and, if all sub¬ 
carriers are operated at the same amplitude, then 
the unmodulated amplitude of each subcarrier 
must not exceed V r /2n. If there is a relatively 
large number of subcarriers not more than 100 
per cent modulated, the likelihood of all of them 
adding up at any one time to give a maximum 
voltage is rather small because of random phases, 
etcA 25,26 However, if an indicating instrument 
should break, a strain-gauge bridge tear off, or 
the like, it is conceivable that a given subcarrier 
might reach a large amplitude unless suitable 
limiting devices are applied (as, for example, 
a limiter tube similar to frequency-modulation 
limiters). However, limiters generally bring in 
harmonics. If the frequency selector is such that 
these harmonics would cause trouble, this can 
be remedied by putting the output of each limiter 
through a filter, which is placed between the 
limiter and the mixer at the transmitter, and 

d This is an important subject in subcarrier telephony. 
There the situation is somewhat improved by single side¬ 
band transmission, allowable occasional cross modula¬ 
tion, etc. 










DESCRIPTION AND TECHNICAL INFORMATION 


9 


which removes harmonics. Figure 6 is a block 
diagram of such a device. A possible alternative 
limiter is outlined later in connection with the 
work of the Rudolph Wurlitzer Company. (See 
Section 1.6.2.) 

In the case of several subcarrier systems under 
development and in use, no provision for limita- 



Figtjre 6. Block diagram of circuit to prevent over¬ 
modulation by use of limiters. 


tion is made because it is felt that periods of 
overmodulation will be so rare as to be unimpor¬ 
tant. 

3. Frequency stability of subcarriers. The sub¬ 
carrier frequencies must be stable enough to 
keep the center frequencies and side bands inside 
the “horizontal” portion of the pass bands of the 
frequency selector. Also, the subcarrier ampli¬ 
tudes must be kept constant to within the re¬ 
quired accuracy of the apparatus. 

4. Phase discrimination. Consider an indicator 
such as a balanced strain-gauge bridge, driven 
by a particular subcarrier frequency. The only 


difference in the output between off balance on 
one side and off balance on the other is that one 
is 180 degrees out of phase with respect to the 
other. Thus, unless provision is made at the re¬ 
ceiver for phase discrimination, there is no way 
of telling on which side the bridge has changed. 
Since a great deal of extra apparatus is required 
to give the time reference for phase discrimina¬ 
tion, it is not provided. Therefore, the bridge 
is operated so that during the measurement the 
balance point is never passed through and the 
bridge is continuously operated off balance on 
one side. Off-balance operation requires twice 
the output amplitudes to yield the same accuracy 
as would be obtained with phase discrimination. 
Thus off-balance operation is not so economical 
as far as the use of the available linear range of 
the radio-transmission link is concerned. 

5. Bridge balancing. In order to accommodate 
a large number of subcarriers with the same ra¬ 
dio link, it is necessary to go to high subcarrier 
frequencies such as 40 or 50 kc. At these fre¬ 
quencies the problem of the capacity balance of 
the bridges often becomes more difficult because 
of long connected cables. This requires cables 
with distributed capacitance and dielectric loss 
adequately independent of temperature, pres¬ 
sure, etc. 

Commutation Systems 

The principle of commutation is to sample or 
to scan a number of channels in sequence, thus 
reducing n channels to one electric channel for 
radio transmission. Figure 7 is a schematic dia- 


FROM n CHANNELS 



TO RECORDING INSTRUMENT 

t t t t_t_t 


U- 

RADIO RECEIVER 


RECEIVER 

H COMMUTATOR 



E j 


1\ _ 


Figure 7. Commutation telemetering equipment. 
















































10 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


gram of commutation telemetering equipment. 

Since the transmitter commutator samples 
each channel, provision must be made at the 
receiver to recreate the intelligence in each chan¬ 
nel from the samples transmitted for that chan¬ 
nel. Smooth sine waves can be reproduced very 
easily from ten samples per cycle. It can be 
shown that if ideal vertical filters could be used, 
a maximum frequency response of one-half the 
number of samples per second could be realized 
by an electric integrator. 21 ' 1 For visual interpre¬ 
tation it is convenient to have about ten samples 
per cycle although a systematic interpretation 
would require fewer samples. The reproduction 
may be done electrically with suitable circuits, 
mechanically, or by drawing smooth curves 
through recorded points. 

During the time a given channel is sampled, 
its intelligence can fill the complete modulation 
range of the radio link. This is an important 
property of commutation as opposed to subcar¬ 
rier systems, in which each channel can occupy, 
on the average, only y 2 n of the modulation 
range. 

Several distinct systems of commutation have 
been developed. 

Television. In the broad sense, television is a 
commutation system because each portion or 
slice of a picture is scanned about 40 times per 
second, which is the framing speed of present- 
day television (Block III). In this system an in¬ 
strument panel containing altimeter, bank and 
turn, tachometer, etc., is televised and photo¬ 
graphed by movie camera at the ground station. 
There are many other methods of indicating 
data, such as projecting light spots from gal¬ 
vanometer elements on the instrument panel so 
that they may be televised. It is, therefore, pos¬ 
sible to get about 40 samples per second (the 
television framing speed) for each of the many 
channels of information. From 40 samples per 
second the theoretical maximum-frequency re¬ 
sponse from the most systematic interpretation 
possible is 40/2 c = 20 c. 14 It has been found that 
about six samples per cycle are required for con¬ 
venient interpretation. Thus a practical fre¬ 
quency response is 40/6 = 6 c. In some systems as 
many as 48 galvanometer spots plus a half-dozen 
flight instruments and 15 breakage indicator 
lights are used. (See Curtiss-Wright systems 


under Section 1.6.2.) However, television is not 
efficient because for a given set of data only a 
small portion of the instrument panel is used 
for the indication and therefore many scanning 
strokes are wasted. For example, one system of 
television uses 15,750 scanning strokes per sec¬ 
ond and produces 30 frames in that time. If each 
stroke were made to indicate one channel of 
information, this would give 15,750/30 = 525 
channels sampled 30 times a second, as compared 
with about 50 channels which are televised at 
present. Thus, television makes use of only 10 
per cent of its capacity. The job of reducing the 
data from 30 to 40 frames of movie film for 
every second is a tedious one and the space (in¬ 
cluding that for optical equipment), weight, and 
power required for television are formidable. 
Against these considerations must be weighed 
the fact that television apparatus is in mass 
production and can be obtained for flight-testing 
purposes. Also, it is likely that in some cases it is 
desirable to reproduce an optical image of flight 
instruments, for example, to observe their be¬ 
havior during maneuvers, observe the horizon, 
etc. For remote-control purposes it may be de¬ 
sirable for the operator to observe a set of instru¬ 
ments which give the common variables such as 
airspeed, altitude, engine revolutions, bank and 
turn indicators, etc. 

This means that improvements as to space, 
weight, power, etc., should be made in television 
apparatus so that it may be used where neces¬ 
sary. Also, more compact and efficient apparatus 
for handling slowly varying channels, now han¬ 
dled by television, should be developed from 
approaches which do not have the fundamental 
limitations of this system. At the receiving sta¬ 
tion, other systems have the advantage that they 
can be made to operate, for example, electric 
meters, which are laid out to resemble flight 
instruments (see Section 1.6.2), as well as to 
operate continuous recording apparatus. 

Direct Commutation. Figure 8 is a schematic 
block diagram of a transmitting commutator. 
Each valve (e.g., a relay, an electronic tube, or 
the like) is turned on and off in sequence. The 
outputs of all valves are connected in parallel. 
First, all valves are off; then No. 1 valve is turned 
on, allowing information only from channel No. 
1 to flow into the transmitter. After one switch- 



DESCRIPTION AND TECHNICAL INFORMATION 


11 


ing period, No. 1 is turned off and valve No. 2 is 
turned on, which allows information from only 
channel No. 2 to flow into the radio transmitter. 
This process is continued until all n channels 



Figure 8. Block diagram for transmitting commutator. 


have been sampled and the sequence starts over 
again. 

The output of the transmitter commutator 
which feeds into the transmitter might appear 
somewhat as in Figure 9A, in which four chan¬ 
nels are indicated. The intelligence in each chan¬ 
nel may be carried by the amplitude or the fre¬ 
quency of the signal sampled from that channel. 
The frequency of each channel relative to the 
commutation may be such that the sampling 
covers many cycles or only part of a cycle. 

At the receiver the channels may be “sorted 
out” and recorded separately or the output of 
the radio receiver may be recorded directly and 
the channels sorted out later, either automatic¬ 
ally or otherwise. If the channels are sorted out 
before recording, there must be some form of 
receiver commutator which is synchronized with 
the transmitter commutator so that when valve 
No. 1 in the transmitter is open, valve No. 1 in 
the receiver is open, etc. (see Figure 9B). The 
synchronization may be carried out by means 
of synchronizing pulses transmitted along with 
the intelligence (as is done in television, for 
example). 

As noted before, the samples per second must 


be somewhat greater than twice the highest fre¬ 
quency to be reproduced. If there are n channels 
to be sampled F times per second, the switching 
speed must be nF per second. The quantity nF 
determines the method of commutation that can 
be used. Several systems will be discussed later. 

Sub commutation. Several installations now in 
use consist of a number of high-frequency chan¬ 
nels telemetered by radio and a number of slowly 
varying channels telemetered by television. As 
mentioned before, apparatus which is smaller, 
lighter, and requires less power can be used for 
handling slowly varying channels for certain 
types of flight testing where optical reproduc¬ 
tions of flight instruments are not required. In 
the case of commutation these slow channels 
could be handled by subcommutation. Figure 10 
is a block diagram illustrating a simple form of 
a subcommutation system. Each subcarrier 
channel is indicated in the shape of a strain- 
gauge bridge, but may be any type of modula¬ 
ting device. The switching frequency of the low- 
speed valves is the sampling frequency of the 
high-speed valves, so that after each sequence 
of these, the low-speed valves switch the excita¬ 
tion to the next column of bridges. The advan¬ 
tage of the square array is that it reduces the 
number of valves (amplifiers, etc.) required. In 
general, if the square contains x channels per 



Figure 9A. Diagrammatic output of transmitter com¬ 
mutator for four channels. 



Figure 9B. Block diagram of receiver commutator. 

























12 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


side, x 2 subcommutator channels can be sampled 
with 2x valves. This becomes important as x gets 
larger. For example, 49 subcommutator chan¬ 
nels could be handled with 14 valves. 

Many other arrangements of subcommutator 
systems are immediately obvious. It would prob- 



Figure 10. Block diagram of subcommutator system for 
many slowly varying channels. 


ably be desirable to make the subcommutator a 
part of the high-speed commutator by devoting 
several high-speed channels to this purpose, thus 
making possible the use of the same radio trans¬ 
mitter. The types to be used should depend upon 
the number of commutation channels to be han¬ 
dled, the number of high-speed channels to be 
devoted to this purpose, space available, etc. At 
the receiver the channel signals can be sorted 
out by a similar, synchronized subcommutator. 
They can then be recorded or, for example, they 
can be made to operate electric meters laid out 
to simulate standard flight instruments, etc. 

Operation and Limitations of Commutator 
Systems. (1) Frequency response of radio link. 
The frequency response of the link between the 
two commutators must be good enough to re¬ 
produce the signal from the transmitter com¬ 


mutator. Suppose that each valve of the com¬ 
mutator is sampling a signal which varies slowly 
enough and that the switching from one valve 
to the next is instantaneous, so that the output 
of the transmitter commutator appears as in 
Figure 11A. In order for 11A to be reproduced 
accurately over a radio link, the frequency re¬ 
sponse of the link must be high enough to re¬ 
produce the sharp corners. For purposes of dis¬ 
cussion let the audio band of the radio link be as 
an ideal low-pass filter with a sharp cutoff at 
frequency F a . If this cutoff, F a is not high 
enough, the corners of Figure 11A will be round¬ 
ed somewhat, as in Figure 11B. The higher (or 
lower) the adjacent channel, the more the round¬ 
ing. Thus intelligence from one channel can 
“spill over” into nearby channels, resulting in 
cross talk. One way to reduce this spilling effect 
is to insert gaps between the channels as in 


A 



n 

( 




^ 

1 

1 1 I 


^ 


B 



Figure 11. Commutator response requirements. 

Figure 11C, in which the dashed lines gives the 
pattern with ideally high-frequency response 
and the solid line the pattern with somewhat 
lower F a . The dotted vertical lines represent the 
time division between channels. The gaps, or 
blank spaces, thus reduce the cross talk if the 


rifiN.FtnW'WTTrrT 


























































DESCRIPTION AND TECHNICAL INFORMATION 


13 


rounded parts of the adjacent channels do not 
cross the dotted lines. This rough illustration 
is to give an idea of the effect of the finite fre¬ 
quency response of the link. 

The important question is how large must F a 
be in order to reduce the cross talk to the re¬ 
quired minimum. The theory and calculations 
required to answer this question are somewhat 
complex, although standard. Consequently, nu¬ 
merical results based on a paper by W. R. 
Bennett 27 are stated as follows. Let correspond¬ 
ing channels at transmitter and receiver be on 
for 1/a of their allotted time, i.e., 1/a of the 
time interval 1/nF. Also, let the output from 
the transmitter commutator be the same type 
as in the dashed lines of Figure 11C. Figure 12 
is a plot of the cross-talk suppression to adjacent 
channels for a set of 20 commutator channels 
plotted as a function of the upper limit F a of 
the radio link in terms of the sampling fre¬ 
quency F (i.e., the ordinate gives the db down 
between channels if the low pass were cut off 
sharply between the integral multiples of the 
sampling frequency on the corresponding ab- 



SIDEBAND PAIRS TRANSMITTED 


Figure 12. Actual cross-talk suppression versus radio¬ 
link frequency characteristics—20-channel commutation. 

scissa). 14 However, the cross-talk suppression 
to any channel cannot be less than the minima 
in the curves of Figure 12 regardless of how 
the low pass is cut off or regardless of over¬ 
lapping of side-band pairs (/ > > F/ 2). Also the 
square corners of Figure 11 require a higher 
frequency response than rounded ones, such as 
portions of sine waves. Figure 13 is a plot of 
cross talk for various a’s, made by drawing a 



10 20 30 40 50 60 70 80 90 


SIDEBAND PAIRS TRANSMITTED 

Figure 13. Minimum cross-talk suppression versus 
radio-link frequency characteristics—20-channel com¬ 
mutation. 

curve through the minimum points on curves 
such as those in Figure 12. Figure 13 applies to 
20 channels. However, if ten channels were used, 
approximate results would be obtained by di¬ 
viding the abscissa values by two. If 40 channels 
were used, the abscissa values should be mul¬ 
tiplied by two and the approximation would be 
more accurate the larger the number of chan¬ 
nels. Thus, from the point of view of cross talk 
and minimizing of F a , if 50 db down (or more) 
is required, it is better to use values of a = 3 
or more. If something of the order of 40 db down 
is required, values of a near unity are better. 

Suppose that large values of a are used and 
that a low-pass cutoff F a could be arranged to 
cut off sharply between adjacent side-band pairs 
which requires that /<F/2. It can be shown that 
if an odd number of channels is used, giving the 
proper phasing of frequency components, the 
cutoff frequency F a can be made quite small. 
For example, with 21 channels the minimum 
cutoff F a could be between the tenth and eleventh 
side-band pairs, resulting in the following table 
for cross modulation as a function of a: 


a 

db down 

3.0 

35 

5.0 

43 

7.5 

50 


It is also possible to cut off between certain 
higher adjacent side-band pairs with somewhat 
better results. 




































14 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


2. Linearity of radio link. Since each channel 
can occupy the whole modulation range during 
the time it is sampled, the link between the input 
to the channels and the output to the recording 
instrument need be only as linear as is necessary 
for the type of measurement to be performed. 
If the technicians reducing the data are willing 
to work from calibration curves, the link can 
be somewhat nonlinear. Overmodulation of any 
channel causes no difficulty in any other channel 
provided the frequency response of the link is 
adequate to reproduce the transmitter commu¬ 
tator output without spilling from one channel 
to the next. In other words, overmodulation does 
not result in cross talk of the nature inherent 
in subcarriers. 

3. Synchronization. If a receiver commutator 
is used, positive synchronization with the trans¬ 
mitter commutator must be provided. In case of 
fading or other interruption, the synchroniza¬ 
tion must be readily restored as soon as the radio 
link again becomes operative. The only way to 
achieve this is through the use of synchronizing 
pulses carried by the radio link or transmitted 
over an auxiliary link. 

4. Signal generator. Only one master device 
at the transmitter end is required to operate the 
valves and to insert the synchronizing pulses 
into the transmitted signal. This same device can 
also be used to excite the bridges, etc. Since the 
receiver commutator is tied in by the synchroniz¬ 
ing pulses, timing of the master device is not 
critical. 

5. Bridge-voltage supply. The amplitude of 
the bridge-driving frequency must be held con¬ 
stant to within the required accuracy of the 
apparatus, etc. 

Combinations of Subcarrier 
and Commutation Systems 

When a number of channels to carry slowly 
varied signals is required, a combination of sub¬ 
carrier and commutation systems can be used. 
Numerous combinations are possible, but only 
one will be mentioned here. This system is very 
similar to the subcommutation in Figure 10 and 
is shown schematically in Figure 14. Subcarrier 
frequency generators continuously excite the col¬ 
umns of bridges. (Any other form of modulator 
could be used.) The commutator valves sample 


each row of bridges in sequence. The receiving 
apparatus consists of a commutator, synchro¬ 
nized with the transmitter commutator, and a 
frequency selector. These act essentially in the 
reverse order to the transmitter commutator. 



Figure 14. Block diagram of subcarrier commutation 
system for many slowly varying channels. 


In this system the switching rate must not be 
so great that the pass bands of the receiver fre¬ 
quency selectors are not adequate; otherwise 
the commutated channels of the same subcarrier 
frequency will spill into each other. The square 
array has the advantage of handling a large 
number of channels with a minimum number of 
valves, subcarriers, amplifiers, etc., as pre¬ 
viously explained. 

A combination of this nature could be incor¬ 
porated into a system of subcarriers which car¬ 
ries rapidly varying channels, making possible 
the use of the same radio link for both rapidly 
and slowly varying channels. 

1,6-2 Summary of Telemetering Systems 
in Use and under Development 

In this section some of the various systems 
recently developed for aircraft telemetering are 
described briefly. They are designated by the 
names of the companies or organizations by 
which they were developed. It is to be understood 
that in most cases the data given are for experi¬ 
mental models. Considerable improvement in 

































DESCRIPTION AND TECHNICAL INFORMATION 


15 


space, weight, power, etc., may be realized when 
the apparatus is engineered for the final models. 

Subcarrier Systems 

Curtiss-Wright Corporation, Buffalo, New 
York. The Curtiss-Wright equipment for struc¬ 
tural flight testing consists of radio and tele¬ 
vision-telemetering systems, synchronously con¬ 
nected. The former has 14 amplitude-modulated 
subcarriers for channels requiring uniform 
(±0.5 db, or about ±5 per cent) frequency re¬ 
sponse up to 200 c. The television system trans¬ 
mits an image of a panel on which are mounted 
six flight instruments and a screen upon which 
are projected light spots from 48 galvanometer 
elements. The television is used for transmitting 
data on strains, pressures, temperatures, posi¬ 
tion of control surfaces, etc., which are not ex¬ 
pected to vary at a rate exceeding approximately 
8 c. The work was carried out under Navy Con¬ 
tract NOa(S)991. 

The subcarrier system is a 14-channel hetero¬ 
dyne type (see Section 1.6.1). The subcarrier 
frequencies are 10,833 to 43,333 in steps of 
2,500 c. e Pertinent data are as follows: 

1. Airborne apparatus. 

a. The 14 subcarrier oscillators are of the 
LC type with compensated feedback to 
assure amplitude stability. The coil and 
condenser combination used showed good 
frequency stability. 

b. The outputs of the oscillators are fed 
through transformers to strain-gauge 
bridges. Their outputs in turn are mixed 
through buffer amplifiers with a common 
plate resistance and then fed to the trans¬ 
mitter through a master amplifier. 

c. The number of tubes per channel is three. 

d. The approximate volume of the airborne 
equipment, exclusive of the power supply 
and radio transmitter, is 2)4 cu ft and 
its weight 55 lb. The weight of the total 
equipment is 144 lb. 

e In a private communication from Curtiss-Wright, it 
was pointed out that these frequencies give 1,288 trouble¬ 
some third-order terms of the type f 1 ±f s ± f 3 and a few 
others. It was further stated that a new selection of 
frequencies had been found that would reduce the 1,288 
to approximately 360. It is probably possible to find a set 
of frequencies which would do even better. The new 
kilocycle frequencies suggested are 6, 8, 10, 14.6, 17.2, 
19.8, 26.4, 30.4, 35.0, 37.6, 41.6, 44.2, 46.8, 48.8. 


e. Excluding the radio transmitter, the B 
power requires 350 ma at 250 v, electron¬ 
ically regulated from a 400-v genemotor. 
The heater requires 7 amp at 22 v. The 
overall power requirement, including the 
needs of the transmitter, is 28 amp at 
22 v direct current, regulated from the 
airplane’s 28-v direct-current power sup¬ 
ply. 

2. Radio link. 

The radio link consists of frequency-modu¬ 
lated transmitter and receiver (Models 1584, 
Fred M. Link Co.). These are operated at ap¬ 
proximately 70 me, with deviation ±75 kc full 
modulation, audio-frequency range 5,000 to 
50,000 c, r-f output power 20 w. The transmitter 
uses the Armstrong system for the frequency 
modulation. 

3. Receiving-station equipment. 

a. The 14 local oscillators are similar to the 
14 airborne subcarrier ones. The 14 crys¬ 
tal filters were manufactured by Western 
Electric Company. Their mid-band fre¬ 
quency is at 92 kc. The attenuation dis¬ 
tortion over a 200-c pass band is within 
±0.25 db. The suppression of all channels 
lying over 2,000 c either side of center 
frequency is over 50 db. The output of the 
crystal filters is amplified, rectified, and 
fed to mechanical recording galvano¬ 
meters. 

b. The approximate weight and size of the 
frequency selector, exclusive of record¬ 
ing galvanometers, radio receiver, and 
power supply, are 600 lb and 17.5 cu ft 
for an experimental unit constructed 
without serious attention to weight. 

c. B power required for the frequency se¬ 
lector is 850 ma at 250 v direct current. 
Heater power is 28 amp at 6.3 v alter¬ 
nating current. Radio-receiver power is 
2.2 amp at 24 v direct current. The total 
power, excluding the needs of the record¬ 
ing galvanometer, is about 750 w. 

Pertinent data concerning the television equip¬ 
ment associated with this system can be sum¬ 
marized as follows: 

1. Model No. 1. S.D.T.E.: Manufacturer — 
Farnsworth Television and Radio Corporation. 




16 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


2. Weight: camera, pressure chamber, and 
radio transmitter — 141 lb; power supply — 25 
lb; cables — 2 lb; instrument panel, including 
six typical instruments and galvanometer block 
—108 lb; total —276 lb. 

3. Volume: camera, pressure chamber, and 
radio transmitter — 5.57 cu ft; power supply — 
0.58 cu ft; instrument panel — 3.18 cu ft; total — 
9.33 cu ft. 

4. Power consumption: television apparatus 

— 33 amp at 22 v; lighting for instrument panel 

— 24 amp at 22 v; total — 57 amp at 22 v. (The 
22 v are regulated down from the 28-v airplane 
power supply.) 

5. Receiver: make — RCA; model — aircraft- 
type CRV-46ABP; volume — 1.42 cu ft; weight 

— 40 lb; power consumption —- 8.9 amp at 25 v. 
Naval Aircraft Experimental Station, Navy 

Yard, Philadelphia, Pennsylvania. This system 
consists of amplitude-modulated subcarriers for 
channels requiring frequency response up to 100 
c, a mechanical commutator and a television unit. 
The subcarrier system, including television, has 
undergone a number of flights, with remote con¬ 
trol, in the F4U airplane. Early models of this 
equipment were manufactured by the Raymond 
Rosen and Company, Philadelphia, Pennsylvania. 
Specifications for a newly engineered model have 
been completed. 

The subcarrier system is a six-channel unit of 
the filter type. (See Ordinary Subcarrier Sys¬ 
tem, under Section 1.6.1.) The subcarrier fre¬ 
quencies are, 3,000; 8,000; 15,000; 25,000; 
29,000; and 49,000 c. These were selected to re¬ 
duce the effects of cross modulation brought in 
by the radio link. 

Pertinent data concerning the subcarrier sys¬ 
tem are as follows: 

1. Airborne apparatus. 

a. The six bridges are fed from the six sub¬ 
carrier oscillators through transformers. 
The outputs of the bridges are mixed 
through buffer amplifiers with a common 
plate resistance and are then fed to the 
transmitter through a master amplifier. 

b. The number of tubes per channel is two, 
exclusive of mixer, master amplifier, and 
transmitter. 

c. The approximate weight and volume of 
the airborne equipment, exclusive of the 


power supply and radio transmitter, are 
39 lb and 0.6 cu ft. 

d. The B power, exclusive of the needs of 
the radio transmitter, is 40 ma at 250 v 
and 40 ma at 150 v, electronically regu¬ 
lated from a 350-v genemotor. The over¬ 
all power requirement, including that 
for the radio transmitter, is 19 amp at 
28 v direct current. 

2. Radio link. 

The radio link consists of a Fred M. Link 
Co. frequency-modulated transmitter using the 
Armstrong system, and receiver, both Models 
No. 1584. These are operated at approximately 
70 me, with deviation ±75 kc full modulation, 
audio-frequency range 5,000 to 50,000 c, r-f out¬ 
put power 20 w. 

3. Receiving-station apparatus. 

a. The filters are of the LC type. The re¬ 
sponse over a ±200-c pass band is flat to 
within 0.25 db. The suppression of fre¬ 
quencies either side of center is 10 db for 
500 c, 40 db or more for 2,000 c. The out¬ 
put of the filters is amplified, rectified, 
and fed to mechanical recording galva¬ 
nometers. 

b. The approximate weight and volume of 
the frequency selector, exclusive of re¬ 
cording galvanometers and radio re¬ 
ceiver, is 65 lb and 2 cu ft. 

c. B power, exclusive of the needs of the 
radio receiver and recording galvanome¬ 
ters, is 30 ma at 250 v direct current and 
20 ma at 150 v. The overall power re¬ 
quirement, including the needs of radio 
receiver, is 6 amp at 28 v direct current. 

For use in connection with its subcarrier 
system the Naval Aircraft Experimental Station 
has a 50-channel commutator system which was 
designed and supplied by Baldwin-Southwark for 
the Navy and manufactured by Raymond Rosen 
Company. The pertinent data for the commuta¬ 
tor system are as follows: 

1. Airborne apparatus. 

a. The 50 bridges of the commutator unit 
are fed from a 6-v Edison storage bat¬ 
tery. The output of each bridge is sam¬ 
pled ten times per second by means of a 
wiping-contact mechanical commutator 
connected at the output of each bridge 



DESCRIPTION AND TECHNICAL INFORMATION 


17 


and driven by an approximately con¬ 
stant-speed direct-current motor. The 
direct-current pulses from the commu¬ 
tator are amplified and then used to 
modulate the 49-kc output of an oscilla¬ 
tor. The modulated pulses are amplified 
and used to modulate the f-m transmitter. 
A 51st bridge having a fixed unbalance 
and a contact duration of twice that for 
the other bridges is used for identifica¬ 
tion and checking gains. This equipment 
was designed for use in place of the sixth 
channel of the Naval Aircraft Experi¬ 
mental Station Subcarrier System. 

b. The number of tubes for oscillator and 
amplifiers is five. 

c. The approximate volume of the airborne 
commutator modulator unit, exclusive of 
that of the Edison battery and power 
supply, is 1.65 cu ft. 

d. The approximate weight of the airborne 
equipment, exclusive of that of power 
supplies and transmitter, is 60 lb. 

e. The power for the bridge circuits is 1.2 
amp at 6 v direct current. The power for 
the commutator motor and modulator is 
6 amp at 28 v direct current. The overall 
power requirement, including the needs 
of the radio transmitter, is 12 amp at 28 v 
direct current. 

2. Radio link. 

The radio link is the same as that for the 
Naval Aircraft Experimental Station [NAES] 
system. 

3. Receiving-station apparatus. 

a. A filter rejects all but the 49-kc signals. 
The output of the filter is amplified, recti¬ 
fied and further amplified. The signal is 
applied to the y axis of a 5-in. cathode- 
ray oscilloscope and to a recording oscil¬ 
lograph which hafc a high-frequency gal¬ 
vanometer. The record produced consists 
of one double-width pulse of fixed ampli¬ 
tude followed by 50 single-width pulses 
of varying amplitude. 

There is no report available as to the perform¬ 
ance, reliability, servicing, etc., of the commu¬ 
tator system. 

There have been no reports issued on the 
NAES television system. This is a special type 


of equipment characterized by compact design, 
light weight, and simplicity of operation. It in¬ 
corporates some of the most advanced engineer¬ 
ing features known to the television art at this 
time. For simplicity, the equipment does not 
conform to the commercial standards established 
for television as to number of lines, frame fre¬ 
quency, or scanning method. Optically, the sen¬ 
sitivity of the camera is about the same as for 
regular commercial equipment. In picture qual¬ 
ity, the special compact equipment suffers 
slightly as compared to commercial broadcasting 
equipment. However, the quality obtained is ade¬ 
quate for applications where fine half-tone 
shading or gradation is not important, as, for 
example, in transmitting images of aircraft in¬ 
strument panels. 

The characteristics of the television equip¬ 
ment used by the NAES in structural flight tests 
are as follows: 

1. Manufacturer: RCA-Victor Compact Tele¬ 
vision or Block Equipment, identified by RCA 
Drawings Nos. M121980, M121098, M120198, 
and M121598. 

2. Weight: camera — 12 lb; camera control — 
15 lb; radio transmitter — 11 lb; power sup¬ 
ply — 31 lb; cables — 15 lb; instrument panel — 
40 lb, including six typical instruments; total — 
124 lb. 

3. Volume: camera — 0.46 cu ft; camera con¬ 
trol — 0.50 cu ft; radio transmitter — 0.48 cu 
ft; power supply — 0.47 cu ft; instrument panel 
— 3.15 cu ft; space front of panel and camera 
lens — 3.74 cu ft; total — 8.80 cu ft. 

4. Power consumption of airborne equipment: 
television apparatus — 22 amp at 28 v; lighting 
for instrument panel — 10 amp at 28 v; total — 
32 amp at 28 v. 

5. Receiver: make — RCA-Victor; model — 
Modified Block III or ARJ Receiver; volume — 
1.87 cu ft; weight — 54.5 lb; power consump¬ 
tion — 10 amp at 28 v. 

C. G. Conn, Ltd., Elkhart, Indiana. The Conn 
system is a subcarrier type which uses the watt¬ 
meter principle for frequency selection. This 
work was done under NDRC contract and was 
discontinued before flight testing. Reports cov¬ 
ering this work are OSRD Nos. 1945 and 3426. 

In the airborne equipment a phonic wheel 
generator driven by a 28-v d-c motor produces 



18 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


14 subcarrier frequencies from 5,000 to 13,500 
in steps of 500 c. The bridges are driven by 
power which comes from the phonic generator 
through transformers. The outputs of the 
bridges are coupled through buffer amplifiers 
with a common plate resistance and are fed 
through a master amplifier to the radio trans¬ 
mitter. In addition to the phonic wheel genera¬ 
tor, only one tube per channel is required. For 
large numbers of channels this type of subcar¬ 
rier frequency generation would be economical 
of space, power, and weight, provided the fre¬ 
quencies could be reliably stabilized. Also, the 
ratios of all subcarrier frequencies are invariant. 

In the receiving equipment the wattmeter 
principle of frequency selection is used. The local 
oscillators consist of a phonic wheel driven by 
a synchronous motor fed by a 500-c signal from 
the transmitter phonic wheel. Considerable 
difficulty was experienced in keeping the syn¬ 
chronous motor from hunting which introduces 
a varying phase angle. This is serious because 
the wattmeter type of response depends upon 
the cosine of the phase angle between the locally 
generated frequency and the subcarrier fre¬ 
quency received from the airplane. Since it is 
assumed that the locally generated frequencies 
are synchronized with the airborne frequencies, 
some drifting of subcarrier frequencies is per¬ 
mitted. (See Section 1.6.1.) The wattmeter prin¬ 
ciple is applied through a square-law type of 
balance vacuum-tube circuit and a low-pass filter 
for each channel. Inasmuch as the wattmeter 
method is phase sensitive, the bridges can be 
operated on balance with subsequent economy 
of modulation range. (See Section 1.6.1.) It 
is quite possible that the wattmeter frequency 
selector could be made to work satisfactorily 
through the use of electronic generation of the 
local frequencies. Synchronization could be 
maintained through an unmodulated subcar¬ 
rier, but constant phase relations must be 
insured. If suitable band-pass filters are avail¬ 
able for the frequency selector, the watt¬ 
meter system as a whole is probably no better 
than the filter system and is somewhat more 
critical. However, this system does not have the 
advantage of phase sensitivity and of locking 
the frequency selector with the transmitted 
frequencies. 


Rudolph Wurlitzer Company, North Tona- 
wanda, New York. The Rudolph Wurlitzer Com¬ 
pany has worked on two types of telemetering 
systems under NDRC contract. The first of these 
was a pulse-modulation subcarrier system for 



Figure 15. Modulation envelope. 


telemetering of slowly varying aircraft flight in¬ 
struments i- (e.g., altimeter and compass) ; the 
second, a system of subcarrier telemetering for 
channels carrying frequencies up to 300 c. 

The pertinent data for the first system are as 
follows: 

1. Airborne apparatus. 

a. For each channel, a Pioneer Magnesyn- 
Autosyn torque amplifier is connected to 



Figure 16. Transmitting instruments for Wurlitzer 
instrument-telemetering system. 

the pointer of each instrument to be tele¬ 
metered, as shown in Figure 16. The out¬ 
put of each torque amplifier drives a po¬ 
tentiometer, shown in Figure 17, which 
is arranged to vary the length of rec¬ 
tangular pulses recurring ten times a 
second. The complete transmitting equip¬ 
ment is shown in Figure 18. In Figure 15 
these pulses are represented by the rec- 







DESCRIPTION AND TECHNICAL INFORMATION 


19 



Figure 17. Precision potentiometer, Autosyn low- 
inertia motor assembly for Wurlitzer instrument¬ 
telemetering system. 

tangular envelope. A subcarrier fre¬ 
quency is assigned to each channel and is 
turned on and off by an electronic valve 
controlled by the rectangular pulses. The 
subcarrier frequencies of one model are 
9.5, 15, 27, 33, and 39 kc. The subcarrier 
output of a given channel appears in 


Figure 15. The length of the pulses de¬ 
pends upon the instrument reading, so 
that the result is essentially an ampli¬ 
tude-modulated subcarrier system. 

b. The transmitter is an f-m Doolittle 
GFY-2. 

2. Receiving apparatus. 

a. The receiver is an f-m Doolittle GVY-4. 

b. The frequency selector employs two reso¬ 
nant-coupled circuits as filters. The rec¬ 
tangular-wave-modulated subcarriers 
are fed to an electronic switch in each 
channel which is on when the pulse in 
that channel is on, and off when the pulse 
is off. When the electronic switch is on, 
it feeds a regulated current through a 
meter which has a long period compared 
to the rectangular-wave period. The 
meter thus reads an average which de¬ 
pends directly upon the duration of the 
rectangular pulses and, therefore, its 
reading corresponds to that of the in¬ 
strument at the airborne end of the same 



Figure 18. Complete transmitting equipment for Wurlitzer instrument-telemetering system. 










20 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 



Figure 19. Receiving instruments for Wurlitzer instru¬ 
ment-telemetering system. 

channel. The meters are 270-degree mil- 
liammeters with the dial of each cali¬ 
brated to match that of the corresponding 
aircraft instrument. This method gives 


a very high signal-to-noise ratio. By the 
sampling rule stated in Section 1.6.1, the 
maximum frequency response of the sys¬ 
tem should be the recurrence frequency 
of the rectangular pulses divided by 2 
or 5 c. This equipment was flight-tested 
by the Aircraft Radio Laboratory at 
Wright Field. 24 

Flight tests at the Naval Aircraft Field 
in Philadelphia were held from July 
through August 1942. 12 The instrument 
showed drift difficulties, thereby requir¬ 
ing frequent calibration. The operation 
of the apparatus was not independent of 
input voltage, temperature, etc. The sat¬ 
isfactory range of the radio link was 
about 20 miles. Some, if not all. of these 
difficulties could certainly have been 
eliminated. However, for flight-instru¬ 
ment telemetering, it seemed more advis¬ 
able to use television, which in the mean- 



Figure 20. Complete receiving equipment for Wurlitzer instrument-telemetering system. 


(«41NF 1 t>F N Ti-AI. 











DESCRIPTION AND TECHNICAL INFORMATION 


21 


time had been developed to the point 
where it could be applied (Block III). 
One obvious advantage of television is 
that no mechanical connections to the 
flight instruments are required. Consid¬ 
ering all these factors, work on this sys¬ 
tem was dropped. 

In July 1943, the Wurlitzer Company com¬ 
menced work on the second system, i.e., sub¬ 
carrier telemetering for channels carrying fre¬ 
quencies up to 300 c. This work did not progress 
beyond the preliminary stage. 13 Four methods 
were proposed: 

1. A subcarrier system of the filter type (Sec¬ 
tion 1.6.1) with a provision for varying the gain 
of the receiving-amplifier circuits by means of 
a monitoring signal carried in an additional 
channel. This would make up for changes in gain, 
etc., in the common parts of the circuits and 
radio link. Work on this plan was discontinued 
while other possibilities were being investigated. 

2. A system which phase modulates the intel¬ 
ligence channels and at the receiver compares 
these phases with a monitoring signal trans¬ 
mitted over an additional channel. This appears 
to be desirable for instrument telemetering simi¬ 
lar to that described in the above paragraphs, 
in which the phase shift is obtained by use of 
transformers and a potentiometer coupled by 
selsyns. It is not well suited, however, for use 
with strain-gauge bridges because of the diffi¬ 
culty in obtaining a sufficiently linear phase shift 
with change of gauge signal. No proposal was 
made for discrimination at the receiving end. 
It was suggested, however, that if the phase- 
shifting frequency were sufficiently low (as 
would be the case in instrument telemetering) 
the carrier frequency could possibly be used as 
a bridge subcarrier and also for strain-gauge 
indications. This would mean that instruments 
could be telemetered over a link already loaded 
with strain-gauge carriers. 

3. A system employing subcarriers which 
would be modulated by a lower-frequency strain- 
gauge bridge carrier (2,000 c). Such a channel 
was built, but it was found that the frequency 
band required to handle the 2,000-c bridge car¬ 
rier was too great to be used with the required 
number of channels and frequency pass of the 
radio link. That is, the outer side bands of the 


frequency-modulated signal extended into ad¬ 
jacent channels. 

4. An amplitude-modulated subcarrier system 
with an amplitude stabilizer on all transmitter 
and receiving channels. The stabilizer was so 
arranged as to limit peak modulation to con¬ 
stant value, without introducing harmonics, in 
such a way as to give amplitude modulation in¬ 
ward as in Figure 21. The peak modulation value 

T| 

e 

L 


A NO MODULATION 



B 50% MODULATION 



Figure 21. Wurlitzer modulation pattern, r-f. 

was accurately stabilized by the use of a voltage- 
regulator tube and would, if working properly, 
compensate for changes in gain between the 
transmitter stabilizer and the receiver stabilizer. 
Work on this system was carried far enough to 
show that it was apparently feasible. 

In all this work the Wurlitzer Company pro¬ 
posed driving the bridges at a lower frequency 
(2,000 c was tried) and modulating higher-fre¬ 
quency subcarriers with the outputs of the 
bridges. For amplitude modulation this requires 
a band width of at least 4,000 c; and for fre¬ 
quency modulation, a band width somewhat 
greater. The Wurlitzer contracts were termi¬ 
nated before further work was accomplished. 














22 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


Boeing Aircraft Company, Seattle, Washing¬ 
ton. As of September 1945, the Boeing Company 
had equipment in an advanced stage of construc¬ 
tion which would be ready for tests in the near 
future. The design is a 20-channel amplitude- 
modulated subcarrier system flat within 0.2 db 
up to 150 c on the lower-frequency channels. 
Band width is greater on the higher-frequency 
channels and is sufficient to permit commuta¬ 
tion with a pulse duration of 6 milliseconds. The 
system will normally be used with 14 channels 
for continuous recording of strains and vibra¬ 
tions up to 150 c, and 6 channels for transmis¬ 
sion of 25 slowly varying signals by means of 
the combined subcarrier commutation system 
described later in this chapter. 

Pertinent data concerning this system are as 
follows: 

1. Airborne apparatus. 

a. Each of the 14 continuous channels con¬ 
sists of an oscillator, gauge balance cir¬ 
cuit, and preamplifier mounted in a 
drawer assembly. These 14 drawers, and 
a 15th for a master amplifier, are housed 
in a box with a controlled fan for cooling 
and temperature regulation. The equip¬ 
ment for the 25 commutated channels is 
described later in this section. 

b. The oscillators are of the LC type with 
close compensation to minimize change 
of frequency or output voltage with tem¬ 
perature. 

c. The outputs of the oscillators are fed 
through balanced transformers to strain- 
gauge bridges or any other suitable type 
of gauge. Resistance and capacitance 
gauge balance controls are provided, and 
the resulting gauge signals are fed 
through midget step-up transformers to 
the preamplifier grids. The 14 preampli¬ 
fier outputs are mixed in a two-stage 
master amplifier and fed from there to 
the radio transmitter. 

d. Two relays are mounted on each drawer 
for the purpose of automatically provid¬ 
ing a zero point and a calibrating signal 
for each channel once each minute. 

e. The number of tubes per channel is three. 

f. The approximate weight and volume of 


the airborne equipment, exclusive of 
power supply and radio transmitter, are 
75 lb and 1.6 cu ft. The total weight of 
the airborne equipment is 136 lb. 

g. B power, exclusive of that required for 
the radio transmitter, is 160 ma at 250 v, 
electronically regulated from a 375-v 
dynamotor. The heater consumption is 
4.5 amp at 28 v and the total power re¬ 
quirement, including the radio trans¬ 
mitter, is 22 amp at 28 v. 

2. Radio link. 

The radio link consists of a frequency- 
modulated transmitter and receiver, Models 
1706T and 1706R of the Fred M. Link Co. 

3. Receiving-station equipment. 

The range of subcarrier frequencies is 
2,000 to 21,360 c. No effort to avoid har¬ 
monic frequencies is made, because it is 
practical to keep oscillator harmonics be¬ 
low 0,5 per cent, and harmonics generated 
in the transmission system are negligible 
when distortion is reduced to the extent 
necessary to minimize cross-modulation 
effects. Three-section LC filters are used, 
each one designed for a different frequency. 
Suppression at 150 c less than the next 
higher mid-band frequency is 45 db and at 
the adjacent mid-band frequency, 53 db. 
The filter output is rectified and fed to 
oscillograph galvanometers with 400-c reso¬ 
nant frequency. 

The weight of the apparatus is not known. 
The approximate volume of the frequency 
selector is 4.6 cu ft. B pow r er required for 
the master amplifier and the rectifiers is 
200 ma at 250 v. The overall power require¬ 
ment, including that for the radio receiver, 
is 1.7 amp at 115 v alternating current. 
Exclusive of oscillographs and power sup¬ 
ply, the receiving equipment occupies 38.5 
in. of standard 19-in. rack. If the radio 
transmitter, the radio receiver, and the 
power supplies are excluded from consider¬ 
ation, the tube count is: 

Airborne: 3 per channel plus 2 for 
master amplifier; 

Receiver: 1 per channel plus 4 for 
master amplifier; 

Total (for 14 channels): 62. 



DESCRIPTION AND TECHNICAL INFORMATION 


23 


Commutation Systems 

The principles of commutation systems are 
not as generally known as are those of subcar¬ 
rier systems, which have been used for many 
years by the telephone companies. In addition, 
their principles are somewhat more complicated 
in detail. For this reason considerably more 
space is devoted to outlines of the commutation 
systems in use and under development than to 
subcarrier systems. 

Julien P. Friez and Sons, Division of Bendix 
Aviation Corporation, Baltimore, Maryland. The 
Friez method employs Radio-Sonde transmit¬ 
ting equipment designed for use in meteoro¬ 
logical balloons. A description of this equipment 
is published in a Julien Friez manual. 31 

At the transmitting end a grid-blocking type 
oscillator is tuned for approximately 1 me. To 
accomplish the blocking, the grid leak is shunted 
by a capacitor. The blocking oscillator is coupled 
into a 72.2-mc oscillator in such a way that the 
latter is on when the former is off, and vice 
versa. The 72.2-mc oscillator is coupled to an 
antenna. The blocking frequency depends upon 
the size of the grid-leak resistance and is varied 
from approximately 8 to 200 c. Intelligence in 
the channels is reduced to resistance variation 
and is inserted by a mechanical commutator into 
the grid circuit, thereby varying the blocking 
frequency. There are essentially four channels: 
one for pressure, one for humidity, and one each 
for high and low reference points. The commu¬ 
tator consists of an arm which wipes over a 
series of points through an arc of 30 degrees. 
The arm is activated by the mechanism of an 
aneroid barometer during the ascent. This ane¬ 
roid switches in the four channels in order. One 
channel of points is connected to a fixed resist¬ 
ance which gives a “low reference point” every 
5,000 ft of ascent and a “high reference point” 
every 15,000 ft of ascent. The temperature and 
humidity channels are suitably sandwiched be¬ 
tween the reference points. 

The transmitted signals are received by a 
special super-regenerative receiver. The audio¬ 
blocking frequencies, varying from 8 to 200 c, 
are fed to a frequency meter connected to a re¬ 
corder which uses a carbon paper tape. 

The blocking type of oscillator may prove to 


be of some convenience in other applications of 
telemetering (involving subcarriers) in which, 
for example, thermistors can be applied in the 
measurement of temperature, etc. 

Consolidated Vultee Aircraft Corporation, 
San Diego, California. A description of the Vul¬ 
tee Radio Recorder has been published in the 
Aeronautical Engineering Review. 29 Several 
improvements have since been made, as de¬ 
scribed here. 

In the airborne apparatus a motor-driven cam 
activates magnetically operated relays with con¬ 
tact points sealed in hydrogen, constructed by 
Electrical Research Products Company. All con¬ 
tact-point terminals are brought out to terminal 
boards where various combinations can be set 
up, such as sampling one channel U0 times a 
second and UO other channels once each second, 
etc. Instrumentation is set up around resistance 
or reactance bridges. Each sample of each chan¬ 
nel includes a number of cycles of the bridge fre¬ 
quency. The output of the commutator is recti¬ 
fied and fed into a variable oscillator having the 
frequency swing of 1,000 to 3,000 c. The fre¬ 
quency of the output is a function of the position 
or reading of the instrument. This is adjusted 
for constant amplitude and fed into the micro¬ 
phone line of a special f-m transmitter. 

On the ground, the signals received are segre¬ 
gated by a special switching arrangement and 
each instrument record is plotted on its own in¬ 
dividual chart. The charts are 6 in. wide, and 
y 16 - in. linear movement represents one second 
of flight. Thus, each instrument has a continuous 
instantaneous plot of readings taken once per 
second. 

Data on the airborne equipment are as follows: 
The dynamotor unit weighs 25 lb and has a 
volume of 0.5 cu ft; the scanning switch, 12 lb 
and 0.75 cu ft; the converter unit, 15 lb and 1 
cu ft; the transmitter, 6 lb and 0.75 cu ft. 

Princeton University, Princeton, Neiv Jer¬ 
sey. 16 - 23 Pertinent data concerning telemetering 
developments at Princeton University under 
NDRC contract are as follows: 

In the airborne apparatus the valves of Figure 
8 are electronic and operate at a sampling rate 
F of 1,000 per second. The number of channels n 
is 20, giving nF = 20,000 per second. Each sam¬ 
pling sequence is initiated by a “master pulse” 



24 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


scaled from switching pulses which operate the 
valves in sequence. These pulses are generated 
from the frequency used to drive strain-gauge 
bridges, accelerometers, etc. Three distinct 
methods of sampling have been developed. 

1. Exemplified by Type I, Model B, illustrated 
in Figures 42 to 48 at end of chapter. The instru¬ 
ments, bridges, etc., are driven at 10 kc and are 
sampled during one-half of a period. Figure 22 
illustrates this method of sampling. The sine 
wave in this figure is the output of a channel, 
e.g., a strain-gauge bridge. The samples of half¬ 
sine waves are taken an even number of half 


/-SAMPLE /— SAMPLE 



Figure 22. Method of sampling in NDRC telemetering 
apparatus designated as Type I, Model B. 


cycles apart. It is clear that such sampling essen¬ 
tially rectifies the output of the channel and, 
therefore, introduces d-c components which must 
be preserved by direct coupling throughout the 
radio link and other circuits. This requires the 
use of a reactance-tube-modulated radio link in¬ 
asmuch as the Armstrong system cannot handle 
direct current. Direct coupling introduces the 
difficulty of drifts due to changes in the static 
characteristics of vacuum tubes and drifts in 
frequency of the radio link. The output of one 

A 


A VALVE OUTPUT 




B COMMUTATOR OUTPUT 


Figure 23. A, valve output; B, commutator output; 
Type I, Model B telemetering apparatus. 


valve might appear as in Figure 23A, and the 
output of the commutator as in Figure 23B. The 
gaps between the channels are used for pulses 
to operate the receiver commutator. 

2. Exemplified by Type II, Model A. The 


bridges are driven at 10 kc, but the outputs of 
the channels are sampled an odd number of half 
cycles apart to give up-and-down sampling as 


/—SAMPLE 



A UP-AND-DOWN SAMPLING '— SAMPLE 



B OUTPUT OF A GIVEN CHANNEL VALVE 

Figure 24. Sampling in NDRC telemetering apparatus 
designated as Type II, Model A. 


in Figure 24A. (See Section under Bell Tele¬ 
phone Laboratories, New York, N. Y., in this 
chapter for application of a rotary-beam tube 
to commutation.) The output of a given channel 
valve appears as in Figure 24B. 

If there were no signal in the channel, the 
output of each valve would be given by a dotted 
flat top. The introduction of the signal into the 
channel results in the up-and-down half-sine 
waves. The dashed sine wave is the lowest fre- 



Figure 25. Commutator output; Type II, Model A, 
telemetering apparatus. 


quency component introduced by the up-and- 
down half-sine waves and has a frequency of 
F/ 2, and an amplitude proportional to the out¬ 
put of the channel. Thus, the sampling does not 
introduce the d-c component as in Method 1. 
This eliminates the necessity for direct coupling 
and the dependence on the drift of the static 
characteristics of the tubes, on the frequency 
drift of the transmitter and receiver, etc. The 
output of the commutator appears as in Figure 
25. The lowest component which the radio trans¬ 
mitter must handle is about 500 c less modula¬ 
tion frequency. 

3. Exemplified by Type III, Model A. It uses 
20 kc on the bridges and samples full sine waves, 
as shown in Figure 26. The output of the com- 





































DESCRIPTION AND TECHNICAL INFORMATION 


25 


mutator is somewhat as shown in Figure 27. 
This sampling does not introduce a d-c compo¬ 
nent. Ideally, the lowest frequency component 
will be the sampling rate F = 1,000 per second. 


/— SAMPLE ,— SAMPLE 



Figure 26. Sampling in NDRC telemetering apparatus 
designated as Type III, Model A. 



Figure 27. Output of commutator in NDRC telemeter¬ 
ing apparatus designated as Type III, Model A. 


The ground-receiver equipment can be sum¬ 
marized in the description of the following 
methods: 

1. The radio transmission is by a frequency- 
modulated link which uses a reactance-tube mod¬ 
ulator to preserve the d-c components. A pulse 
extractor connected to the output of the discrim¬ 
inator is used to extract switching pulses from 
the gaps between the channels (illustrated in 
Figure 23). The master pulse is transmitted by 
cutting off for an instant one of the multipliers 
in the transmitter, which cuts off its radiation. 
This creates a pulse in the grid of one of the 
limiters in the receiver. The switching pulses and 
master pulses so carried over the radio link are 
used to drive the receiver commutator. In this 
way it is positively locked in with the transmitter 
commutator and achieves synchronization in 
1 millisecond. The output of each receiver valve 
(see Figure 9B) has the samples formed into a 
smooth signal and fed to recording galvano¬ 
meters. 

2. The radio link, pulse selector, etc., can be 
the same as in Method 1, except direct coupling 
is not used. The output of each receiver valve 
is fed through a low-pass filter to carry the 500-c 
component and cut off everything above. This 
component is then rectified and fed to the re¬ 
cording galvanometers. This system is not phase 
sensitive. (See Section 1.6.1.) 

3. The sampling is designed to be used with a 
cathode-ray recording device. Each channel is 


provided with its own cathode-ray tube on which 
the complete signal, received from the discrim¬ 
inator, is spread out by a sweep generated from 
the master pulse which is carried in a manner 
analogous to the switching pulses in Methods 1 
and 2. No switching pulses are required. Figure 
28 shows the signal on a cathode-ray tube. A 
mask, which is opaque except for a slit, is put 
over the tube and the centering is adjusted to 
select the desired channel. The tube is then 
photographed on moving film, which moves more 
slowly than the sweep. The envelope of the 
samples recreates the envelope in the output of 
the channel before it was sampled. The intelli¬ 
gence is measured from tip to tip. This method 
is phase sensitive because the slope of the con¬ 
necting lines is positive or negative depending 
upon whether, for example, a strain-gauge 
bridge is stretched or compressed. This method 
of sampling and recording is independent of 



Figure 28. Signal from single channel chosen by mask 
over cathode-ray tube (NDRC telemetering apparatus 
designated as Type III, Model A). 


drifts over periods of time longer than the period 
of bridge-driving frequency. In Model A, Type 
III, currently being developed, the cameras are 
so arranged as to photograph three channels to¬ 
gether with timing, etc., on a 35-mm film. Six 
films are required to photograph 18 channels. 
The Armstrong system, which is crystal con¬ 
trolled, can be used in the radio link. 
















26 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


The airborne apparatus is essentially the same 
for the three methods except for differences in 
exciting frequencies and the scaling ratio be¬ 
tween the master pulses. The receiving appara¬ 
tus is different for the three systems. Table 2 
gives a comparison of the receivers. The number 
of tubes stated does not include those in the radio 
receiver or in the voltage supplies and regula¬ 
tors. 

Inasmuch as Method 1 requires no filters and 
was developed before the other methods, it was 
incorporated in prototype apparatus and flight 
tested 19 in the YP-59 jet-propelled aircraft at 
the Bell Aircraft Corporation at Niagara Falls. 
Automatic calibration provisions are being built 
into the airborne commutator to give zero and 10 
calibration points on each channel in sequence. 
The complete operation will require about 15 
seconds and can be initiated by a single pulse 
from the remote-control apparatus. Relays have 
been set up to calibrate before and after each 
maneuver, to give a complete check on the ap¬ 
paratus. This eliminates the difficulty of slow 
drift which is hard to remove entirely from the 
direct-coupled circuits and radio link. 

The equipment used in the YP-59 is shown in 
Figures 29 to 31 inclusive. Its physical charac¬ 
teristics are as follows: 


1. Airborne equipment. 

a. Eighteen channels — one channel used 

for zero level. 

(1) Instrumentation includes 13 strain- 
gauge bridges, two accelerometers, 
and one elevator position indicator. 

(2) Driving frequency of bridge, etc., 
10 kc. 

(3) Sampling rate, 1,000 per second. 
Switching rate, 20,000 per second. 

(4) Average number of tubes per chan¬ 
nel is four (exclusive of radio trans¬ 
mitter). 

b. Weight and volume components. 

(1) Commutator amplifier unit 

75 lb 2 cu ft 

(2) Voltage regulator-filter 

25 lb 0.5 cu ft 

(3) Radio transmitter 15 lb 0.5 cu ft 

(4) Two dynamotors 20 lb . . . 


(5) Total 135 lb 3 cu ft 

c. Power requirement. 

(1) B power, 400 ma at 250 v direct cur¬ 
rent, regulated from 450 v direct 
current. 

(2) Total power, 35 amp at 28 v direct 
current. 


Table 2 


Method 

Number 
of tubes 
per 

channel 

Total 
number 
of tubes 

Frequency band required of 
radio link and associated cir¬ 
cuits to keep cross talk down 
50 db. 

Drift due to frequency. 

Drift of radio link, shift in 
static characteristics of tubes, 
etc. 

Recording 

instrument 

1 

5 

107 

0 - 70,000 c 

Dependent except for fre¬ 
quency clamping circuit used 
to regulate frequency of local 
oscillator to keep constant 
output of an unmodulated 
channel. 

Mechanical 

multi¬ 

channel 

oscillo¬ 

graph 

2 

6 

125 

400 - 70,000 c 

Independent of slow drifts 
which are eliminated by coup¬ 
ling networks. 

Mechanical 

multi¬ 

channel 

oscillo¬ 

graph 

3 

1 

cathode- 
ray tube 

16 tubes 
plus 18 
cathode- 
ray tubes 

1,000 - 70,000 c 

Independent of drifts. 

18 cathode- 
ray tubes 
photo¬ 
graphed 
on six 
35-mm 
films 













DESCRIPTION AND TECHNICAL INFORMATION 


27 


2. Radio link. 

a. A frequency-modulated radio link de¬ 
veloped at Princeton and constructed by 
Fred M. Link Co. is employed. 

b. The modulation is obtained by use of a 
reactance tube to give a frequency swing 
of ±125 kc with a deviation ratio of 
approximately 1.5. A carrier frequency 
of 69 me is obtained by beating the re¬ 
actance-modulated frequency of 3 me 
against a crystal-controlled frequency of 
72 me. The r-f power output is about 30 w. 
A quarter-wave horizontal antenna is 
used. The standing-wave ratio on the 
feeder coaxial cable is about 2. 

c. The receiver uses two intermediate fre¬ 
quencies and a double limiter. The dis¬ 
criminator is arranged for a linear out¬ 
put with a maximum of 60 v peak to peak. 

3. Ground equipment. 

a. The receiver commutator is synchronized 
with the transmitter commutator by 
means of pulses carried by the radio link. 


The samples are reassembled electrically 
and fed into a Consolidated recording 
galvanometer. Figure 33 is a typical rec¬ 
ord. The frequency band of each channel, 
exclusive of galvanometer, is flat from 
0 to 200 c. 

b. Full modulation gives about 4 in. peak- 
to-peak deflection on the oscillograph. 

c. The average number of tubes per chan¬ 
nel is six, exclusive of the radio receiver. 

d. The overall weight of the receiving ap¬ 
paratus is about 400 lb. 

e. The overall power requirement is 50 amp 
at 28 v direct current. 

Bell Telephone Laboratories, New York, New 
York. The Bell Telephone Laboratories [BTL] 
have developed a magnetically rotated-beam 
tube, making possible the use of many grids and 
anodes for electronic commutation. 28 - 30 Features 
of the rotary-beam tube are its small size, 
low voltage, linear characteristics, and high- 
beam currents. A rotating magnetic field to drive 
the tube can be readily obtained by the use of 



AMPLIFIER 

COMMUTATOR 


VOLTAGE 

REGULATOR 


-POWER SUPPLY 
OYNAMOTORS 


^RAOIO 

TRANSMITTER 


Figure 29. Front view of telemetering equipment in YP-59. 














28 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 



Figure 30. Rear view of telemetering equipment in A’P-59. 


the stator of a small polyphase alternating-cur¬ 
rent motor. Single-phase power can be split for 
this purpose. At low rotating frequencies the 
power loss is mostly in the copper of the wind¬ 
ings, but at higher frequencies core loss becomes 
important. It should be possible to rotate the 
beam at 1,000 c using just a few watts. 



Figure 31. Telemetering antenna (long one). 


The original papers 28 - 30 describe a 80-anode 
tube which could be used to commutate 30 chan¬ 
nels. This tube, shown in Figure 32 is approxi¬ 
mately 214 in. in diameter and 6 in. long. The 
tube has a single control grid surrounding the 
cathode and a suppressor grid in front of each 
anode. For the airborne commutator the anodes 
can all be connected together and the suppressor 
grids connected to each channel. These modu¬ 
late the beam in succession, similar to the action 
of the Princeton commutator which uses stand¬ 
ard receiving tubes. At the receiving station the 
rotating beam is synchronized with the airborne 
tube, and all suppressor grids are tied together. 
The modulation is applied to the grid surround¬ 
ing the cathode and each anode forms a channel 
output. 

This tube could be used very well with cathode- 
ray recording by Method 3 under the Princeton 
system, or with modifications of Method 2. It 
would probably be desirable to use one half of 
the rotating beam frequency to drive the instru- 














DESCRIPTION AND TECHNICAL INFORMATION 


29 



Figure 32. 30-anode magnetically rotated beam tube 
(top view). 

mentation and to phase each channel so that the 
sampling takes place at the peaks of the sine 
waves in an up-and-down method. This requires 
minimum frequency response of the radio link, 
eliminates d-c components and permits low oper¬ 
ating frequencies of the bridges, thus minimizing 
the effects of distributed capacitance, etc. The 
cathode-ray trace would appear somewhat as in 
Figure 28, with much flatter peaks. 

The BTL have also developed a cathode-ray 
tube which has many anodes. These are divided 


by little stalls which make possible modulation 
by use of the secondary-electron characteristics 
of the anodes. An intensity grid modulating the 
cathode-ray beam is also provided. The tube is 
very compact and requires practically no power 
to sweep the beam. The plates can be arranged 
in a circle for rotary scanning or in rows for 
television-type scanning. This type of tube can 
be used in the same general way as the magnetic¬ 
ally rotated beam tubes. 

Both types of the tube initially produced at 
BTL were further developed under a subcon¬ 
tract with Princeton University to allow appli¬ 
cation to commutation telemetering.The system, 
known as Type II, Model B, 23 shows excellent 
possibilities for extreme compactness, more 
channels (about 30), low power consumption 
and small weight. 

Combination Subcarrier- 
Commutation Systems 

Boeing Aircraft Company, Seattle, Washing¬ 
tonA- The pertinent data of this system are: 

1. Airborne apparatus. 

a. The Boeing system makes use of an array 
of five oscillators and six commutator 
valves arranged as in Figure 14 to give 
30 channels. One of the six channels on 
each oscillator is used to transmit a signal 
which controls the gain of an automatic 
volume control amplifier on the ground. 
Thus, 25 channels are available for trans¬ 
mission of intelligence. Provision is made 
for different types of gauges, including 
wire strain gauges, magnetic gauges and 
slide wires. 

b. The commutator is electronic and has a 
switching rate of 120 per second and a 
sampling rate of 20 per second. A sixth 
oscillator is amplitude-modulated at 120 c 
for transmission of switching and syn¬ 
chronizing signals to the ground. Auto¬ 
matic calibration is provided in each 
channel once each minute, and gives a 
zero input signal and one calibration 
voltage. 

c. The total number of tubes, exclusive of 
those in the transmitter and the power 
supply, is 59. 

d. The volume of the airborne equipment, 








30 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


exclusive of power supply and the trans¬ 
mitter, is 4.4 cu ft. Its weight is 175 lb. 

e. B power, exclusive of that required for 
the radio transmitter, is 200 ma at 250 v, 
electronically regulated. The overall 
power requirement, including that for 
the radio transmitter is 26 amp at 28 v 
direct current. 

2. Radio link. 

Fred M. Link Company Models 1584 are 

used with frequency-modulated transmitter 

and receiver. 

3. Receiving apparatus. 

a. The receiver output drives five amplifiers 
which feed five two-section LC filters. 
The gain of these amplifiers is regulated 
by the automatic volume control signals. 
These amplifiers are also used for blank¬ 
ing between pulses and for rounding the 
pulse tops to reduce the necessary filter 
band width. 

b. Separation of channels is accomplished 
by a square array, essentially the same 
as in Figure 14, except that five filters 
replace the subcarrier generators while 
rectifiers and galvanometers replace the 
bridges. That is, the circuit works in 
reverse. 

c. The electronic commutator is operated 
by pulses derived from the 120-c modu¬ 
lation on the sixth subcarrier. This sub¬ 
carrier is blanked out in the transmitter 
during every switching interval in order 
to provide a master synchronizing pulse. 
In this way the transmitting and receiv¬ 
ing commutators are automatically 
brought into step after any interruption 
of the radio signals. 

d. The signal received by each galvanome¬ 
ter rectifier consists of 20 pulses or bursts 
of the subcarrier per second. Each pulse 
lasts about 6 msec. 

e. The galvanometers are fast enough to 
follow the envelope of the individual 
pulses, so it is not necessary to use an in¬ 
tegrating circuit. Low paper speed is 
used, and the record appears as a succes¬ 
sion of dots. Calibrating signals are re¬ 
corded automatically along with the 
gauge signals. 

f. The total number of receiving tubes, ex¬ 


clusive of those in the radio receiver and 
the power supply, is 98. This number can 
be reduced to 68 if the automatic volume 
control, which is only a convenience, is 
removed. 

g. The receiving equipment, exclusive of 
the radio receiver and the oscillographs, 
occupies 72 in. of 19-in. rack space. 

h. The overall power consumption, includ¬ 
ing that of the radio receiver and the 
three oscillographs, is 15 amp at 115 v 
alternating current. 

Discussion of Test Results 

The subcarrier systems which have been de¬ 
scribed developed cross-modulation difficulty 
under certain conditions, as explained in the 
introduction to this report. Numerous modifica¬ 
tions were made in placement of subcarrier fre¬ 
quencies in relation to one another and in de¬ 
velopment of appropriate band-pass filters to 
act as frequency selectors. Final results led to 
the conclusion that the development’of a more 
linear radio link was essential. Accordingly, at 
the suggestion of the various groups concerned, 
the Navy Department early in 1945 issued a di¬ 
rective to NDRC (NA-226) for assistance in the 
development of an appropriate radio link. This 
link was to permit equipment already developed 
to be used to transmit indications by subcarriers. 
It is probable that such a link could have been 
developed and constructed, thereby eliminating 
some of the difficulties in subcarrier operation. 
However, work was terminated on this project 
with the cessation of hostilities. 

Flight tests of the electronic-commutation 
apparatus Type I, Model B developed under 
NDRC, were carried out in the YP-59 jet-pro¬ 
pelled aircraft and in a Curtiss Helldiver Type 
SB2C-3. The latter test was under the super¬ 
vision of Princeton University personnel associ¬ 
ated with Contract OEMsr-1037. Figure 33 
shows a typical oscillograph record transmitted 
from a 3.5 g pullout in the Helldiver. A number 
of channels, whose indications are represented 
by the heavy lines on the chart, were connected 
to strain-gauge bridges in the aircraft. The 
gauges were mounted on various wing spars 
which show stress under various conditions of 
maneuver of the aircraft. Two other channels 
were used to indicate air speed and acceleration 



DESCRIPTION AND TECHNICAL INFORMATION 


31 


of the aircraft in directions parallel to and nor¬ 
mal to its main axis. This particular set of 
records, at its start, shows flight conditions 
wherein the various stresses and accelerations 
are relatively constant, except for engine vibra¬ 
tion which appears in the accelerometer channel 
to a considerable degree. These normal condi¬ 
tions are for flight in a relatively steep dive with 


to permit Service groups to obtain the apparatus 
needed for their applications. Pictures of this 
apparatus are given in Figures 42 to 48 inclusive, 
reproduced with the circuit diagrams in Section 
1.7 of this report. 

To illustrate the general features of this equip¬ 
ment, and installation details, circuit drawings 
are reproduced in Section 1.7. 



Figure 33. Sketch of oscillographic record of typical 3.5<y pullout, transmitted from project test aircraft SB2C-3 by 
NDRC telemetering equipment Type I, Model B. 


the airspeed increasing slowly because the ter¬ 
minal velocity had almost been reached prior to 
the portion of the record shown. When the record 
is about 30 per cent completed, the pullout from 
this dive is begun with the associated decrease 
in airspeed, increase in some wing loadings and 
decrease in others, and a large increase in accel¬ 
eration. The end of the chart shows the condi¬ 
tions on returning to level flight. The record 
adequately illustrates the application of tele¬ 
metering to the transmission of strain and accel¬ 
eration data from an aircraft to ground by a 
radio link. 

Results indicated that commutation telemeter¬ 
ing in the form tested is satisfactory and will 
adapt itself to the addition of more channels by 
subcommutation or, where high-intelligence fre¬ 
quency response is desired, to the use of addi¬ 
tional switching channels employing rotary- 
beam tubes with multiple targets. As a result 
of the various tests, preproduction prototypes of 
Type I, Model B, were manufactured by another 
contractor (Raymond Rosen and Company, 
OEMsr-1399) under the supervision of NDRC, 


The Type II, Model A, Type II, Model B, and 
Type III, Model A systems described in the body 
of this report were approaching the prototype 
production stage in October 1945, when NDRC 
transferred the contract with Princeton Univer¬ 
sity to the Navy Department. 

Guided-Missile Telemetering 28 

The telemetering developments which have 
been described are adaptable in various forms 
and combinations to the transmission of infor¬ 
mation from guided missiles. In the case of such 
vehicles, telemetering is essential, since there are 
no pilots to obtain any sort of performance data. 
Using the principles established in previous 
telemetering studies, work was started on the 
development of various combination systems for 
use in guided missiles under development by the 
Army and Navy. This work was under LARK 
directive NA-242. When the contract with 
Princeton University was transferred to the 
Navy Department, work was being continued 
under that contract on the development of such 
apparatus. 


CONF1DFN-T4A4. 






























































































32 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


17 CIRCUIT DRAW INGS AND PICTURES - NDRC, TYPE I, 
MODEL B TELEMETERING EQUIPMENT 



I2SN7 12 AH 7 I2SN7 

VT 3 VT ~ 2 SHIELDED VT 1 



AMPH 83-1R AN-3I02-14S-2P 

FRONT PANEL FRONT PANEL POWER 
SIGNAL OUTPUT 


AN-3I02-14S-5S 
FRONT PANEL 
CALIBRATION 


AMPH 83-1R 
FRONT PANEL 
MASTER PULSE 


Figure 35. Transmitter channel No. 2 (NDRC Type I, Model R). 




REAR OF CHASSIS AN - 3102-22‘195 AN-3102-22-19 S REAR OF CHASSIS 




























































































































































































































"15 VOLT BIAS 
SIGNAL IN 
+ 250 VOLTS 
M PULSE OUT 
M PULSE IN 


NDRC, TYPE I, MODEL B TELEMETERING EQUIPMENT 


33 


VT-1 VT-2 VT-3 

I2SL7 I2AH7 I2SN7 



VT-1 VT-2 VT-3 VT-4 VT-5 

12SN7 12SN7 12SL7 12SL7 12SN7 



Figure 37. Receiver commutator channel (NDRC Type I, Model B). 




AN-3102-22- I9S 

REAR OF CHASSIS 

















































































































































































































































34 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 



O 

O 

o 

8 

o 

o 

o. 

II 

* 


It 

o 

CVJ 



a 

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>> 


o 

M 

55 


< 

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as 

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IN PRC, TYPE I, MODEL B TELEMETERING EQUIPMENT 


35 



22 


Q 


a> 


W 



HEATER CIRCUIT 














































































































































































36 


TELEMETERING OF STRAIN GAUGES AND INSTRUMENTS 


R F OUTPUT 
AMPH 83-IR 


3-54 

UMf xr —II-, 


12SH7 
V 2 


I2SH7 

VI 



NOTE: 

"A" 59 TURNS 10 TURN TAP - DIA NO. 28 DSC 
"B" SHIELD OF "SIGNAL OUTPUT" CO'AXIAL 
CONNECTOR INSULATED FROM CHASSIS 
GROUND COMPLETED AT COMMUTATOR 


SELECTOR SWITCH 


Figure 40. F-M transmitter, Model 1790 (NDRC Model I, Type B). 



Figure 41. Front view of airborne commutator with 
dust cover removed (NDRC Type I, Model B). 


Figure 42. Rear view of airborne commutate 
pulse-generator assembly (NDRC Type I, Mod 


and 

B). 

















































































































































































































Figure 43. F-M radio receiver, Model 1789 (NDRC Type I, Model B) 


































































































































































































































































































































































NDRC. TYPE I. MODEL B TELEMETERING EQUIPMENT 


37 



Figure 14. Pulse generator (NDRC' Type I, Model B). 



Figure 45. Airborne voltage regulator (NDRC Type 1, 
Model B). 






Figure 4G. Receiver voltage regulator (NDRC Type I, 

Model B). Figure 48. Receiver channel (NDRC Type I, Model B). 












Chapter 2 


THE ACOUSTIC FIRING ERROR INDICATOR a 

By J. W. M. Du Mond h and E. R. Cohen b 


INTRODUCTION 

nder pressure of demand from three 
branches of the Armed Forces, research 
and development on the acoustic firing error 
indicator [FEI] and its associated auxiliary 
testing and standardizing equipment continued 
actively up to the termination date of the original 
Contract OEMsr-600, October 31, 1945. The 
progress of the work rendered nearly all interim 
and special reports more or less obsolete shortly 
after their appearance so that at the termina¬ 
tion of the contract the need was imperative 
for a comprehensive report bringing all phases 
of the development strictly up to date if the value 
of the results achieved was not to be lost. For 
this reason, every effort has been made in the 
present report to furnish a complete and reli¬ 
able source of all information which might be 
helpful (1) to administrative military and civil 
officials, (2) to the using Services, (3) to re¬ 
search groups entrusted with continuation of 
research and development work, and (4) to per¬ 
sonnel of manufacturers of the equipment. 

For the nontechnical reader the brief resume 
following this introduction gives the salient 
facts at a glance. For his further information 
Sections 2.3, 2.4, and 2.5 have been written with 
a minimum of mathematical analysis and tech¬ 
nical detail. Parts of Section 2.4, “Depend¬ 
ence of Shock-Wave Pressure Amplitude on 
Miss Distance” and “Dependence of Shock- 
Wave Period T' on Range (Projectile Velocity) ” 
might be omitted on a first reading by such read¬ 
ers. Section 2.6 is nonmathematical and descrip¬ 
tive, but treats the FEI equipment and its func¬ 
tioning in considerable technical detail. Section 
2.7 is purely historical. 

a AC-46, NO-173, NO-260. 
b California Institute of Technology. 
c The appendices, to which references are made 
throughout the chapter, are the first five items in the 
bibliography associated with this chapter. As the appen¬ 
dices were voluminous it was decided to microfilm them 
for reference rather than to print them in this volume. 


The mathematical analysis has been kept al¬ 
most entirely in the appendices. c Much of the 
technical information which would be required 
by manufacturers’ personnel is placed there and 
also in Section 2.6 of the main body of the report. 
Appendix V is a collection of descriptive material 
on auxilliary standardizing and measuring 
equipment not described elsewhere in the report. 
This appendix will, therefore, be of interest to 
research workers who may continue the work, 
and also to some extent to the interested Services 
of the Armed Forces. The auxiliary measuring 
and standardizing equipment when built was 
accompanied by instruction manuals which were 
not formal NDRC reports. 33 - 35 ' 38 

22 RESUME 

Purpose of Research 

To develop a means of (1) scoring a gunner as 
to his marksmanship, and (2) informing a gun¬ 
ner (or his instructors) as to his errors when 
shooting at airborne targets such as towed flags, 
towed gliders or radio-controlled planes. 

Basic Method 

The amplitude of the ballistic shock wave 
propagated from the trajectory of the bullet is 
measured by a dual microphonic element in¬ 
stalled in the target, and the result is trans¬ 
mitted by radio to a receiving station (near the 
gun), where the shots are automatically classi¬ 
fied into three different zones of proximity to 
the target and so recorded either on a tape or 
on counters. The side (right or left, fore or 
aft) of the moving target on which the miss 
occurred is also indicated and recorded. 

Limitations of Applicability 
of Present Device 

1. Calibers: .50, 20 mm, 40 mm. 

2. Bullet speeds at the target: 1,400 fps and 
higher. 



38 



INTRODUCTION AND MILITARY REQUIREMENTS 


39 


3. Targets: Velon plastic flags and towed 
gliders. 

4. Target speeds: up to 250 mph. 

5. Radio reception distances (target to re¬ 
ceiving station): cannot exceed 4 miles. 

6. Weather conditions: microphones rendered 
inaccurate by rain or ice formation. 

For other and more detailed limitations see 
Section 2.5.5. 

Validation Results 

Validation tests checking the method against 
photographic theodolite data showed an ac¬ 
curacy in scoring miss distance of ±7 per cent 
on 537 rounds fired at Ft. Bliss, Texas. 

23 INTRODUCTION AND MILITARY 
REQUIREMENTS 

Origin of the Need for a 
Firing Error Indicator 

The greatly increased importance of the air¬ 
plane in World War II has created a need for a 
method of indicating the errors of fire of 
marksmen and their equipment shooting at air¬ 
borne targets both in ground-to-plane and in 
plane-to-plane shooting. The origin of the work 
on the present firing error indicator (FEI) was 
not, however, in a formal Service request for 
the device. It came as the result of certain ideas 
originating with members of the staff of the 
California Institute of Technology who had 
witnessed Army target practice in connection 
with other war research work. Their attention 
was thus forcibly called to the existence of the 
problem which may be stated as follows. 

The aerial targets used in training gunners 
for World War II have necessarily been of 
rather small size, chiefly flags, sleeves, or glid¬ 
ers. These are towed at the end of a consider¬ 
able length (several thousand feet) of cable by 
a special towing plane (usually a B-26) pro¬ 
vided with a large motor-driven winch for 
reeling the cable, and a hatch in the bottom of 
the fuselage just to the rear of the winch, for 
launching the target after the towplane is in 
flight. To some extent small radio-controlled 
planes such as the OQ and PQ have also served 
as training targets. The number of actual hits 
which can be made on such targets is such an 


extremely small fraction of all rounds fired 
that no statistically significant information for 
comparing the marksmanship of different gun¬ 
ners or gun crews, or the relative merits of dif¬ 
ferent kinds of equipment can be based there¬ 
on; nor can much reliable information be fur¬ 
nished (by observing holes in the target after a 
mission) to assist gunners materially in cor¬ 
recting their errors of fire. Experience has also 
shown that visual observation of tracer bullets 
fired at airborne targets is in general an un¬ 
satisfactory method either for training or for 
rating gunners.* 1 The central difficulty in ob¬ 
serving tracer fire is to determine when the 
tracer bullet has reached the range of the 
target, and this is especially true if the ob¬ 
server is at or near the gun. 

2 3 2 The Importance of Training Gunners 
on Targets Permitting Large Scores 

The fact remains that the two problems (1) 
of improving marksmanship by furnishing re¬ 
liable and, if possible, instantaneous informa¬ 
tion as to errors of fire, and (2) of rating gun¬ 
ners and equipment with statistical reliability 
as to marksmanship performance, are of su¬ 
preme importance in warfare. Even a small 
superiority in weapons or men, if it is really 
significant, cannot be ignored since it can be 
reflected as a relatively much larger advantage 
in actual combat, especially in situations which 
partake of the nature of a duel in which the 
more proficient survive. It is not only more 
humane to the gunners but far more efficient 
in the prosecution of the war if the best gun¬ 
ners and the best equipment can be selected 
with as much statistical certainty as possible. 
This seems a far more important consideration 
in scoring gunners and equipment than is the 
frequently heard proposition that the target 
on which the gunners are scored should dupli¬ 
cate in size the targets to be hit under combat 

d Under certain special conditions as to course flown 
and observer position (situated at some distance down 
course from the gun) a special method worked out by the 
AAAS for ground-to-air firing permits visual estimates 
of the firing error in lead and lag by observation of the 
apparent hook in the tracer trajectory. However, this can 
hardly be regarded as a quantitative scoring device, and 
the fact that the use of tracer fire is regarded as a 
detriment in teaching marksmanship makes its applica¬ 
tion somewhat undesirable. 




40 


THE ACOUSTIC FIRING ERROR INDICATOR 


conditions. The problem of classifying gunners 
or rating their equipment is one of selecting 
those which are statistically more likely to give 
hits on the real target, and such a selection can 
be made more accurately and with fewer 
rounds if scoring targets are used which are 
considerably larger than targets usually simu¬ 
lating the size of those used in combat. It can 
be shown 30 * that targets of such size as to 
permit scores from 50 to 80 per cent hits afford 
the best accuracy (as to freedom from statis¬ 
tical fluctuation) in rating marksmanship per¬ 
formance. 

It should be evident to anyone that targets 
so small that scores of only a few hits per thou¬ 
sand rounds can be registered, and targets so 
large as to permit only a few misses per thou¬ 
sand rounds are equally undesirable since such 
occasional hits or misses are likely to be statis¬ 
tical accidents rather than measures of average 
performance. Clearly then there must be, be¬ 
tween these extremes, some optimum size of 
target. Analysis made under statistical laws 
shows that targets permitting scores of the 
order of 80 per cent are to be preferred for 
rating gunners on a relative scale of marks¬ 
manship as to their most probable miss dis¬ 
tance. Targets permitting 50 per cent scores 
are nearly as good, but a 1 per cent target re¬ 
quires the firing of 60 times as many rounds 
for the same reliability as an 80 per cent 
target. 

To tow a material target through the air at 
realistic speeds with sufficiently large dimen¬ 
sions to permit 80 per cent direct hits on it at 
normal working ranges is a practical impos¬ 
sibility. One of the chief advantages aimed at 
in the acoustic FEI is to furnish sensitive zones 
around the target center within which the pas¬ 
sage of a shot will be indicated, such zones be¬ 
ing large enough to permit the 50 to 80 per 
cent hit requirement. 

233 Operating Principle of the Acoustic FEI 

The method used in the present FEI to ac¬ 
complish this result relies on the fact that pro¬ 
jectiles, as long as their speed relative to the 
air mass substantially exceeds the speed of 
sound, send out from their trajectories acoustic 
waves known as ballistic shock waves. The in¬ 


tensity of these shock waves at a given point 
is a diminishing function characteristic of the 
distance of the point from the trajectory of the 
bullet. A microphonic element, known as the 
FEI transmitter and mounted in the airborne 
target, has been developed which sends a quan¬ 
titative signal to the FEI receiving station in¬ 
dicative of the intensity of the shock waves from 
the bullets passing in the vicinity of the target. 
This signal is interpreted at the receiving sta¬ 
tion by automatic means into information re¬ 
garding the proximity of the round to the tar¬ 
get. Since two oppositely directed microphones 
are used in the FEI transmitter, indications 
can also be furnished regarding the direction¬ 
ality (i.e., angular direction) of the misses, 
though such indications are only reliable under 
certain limitations as to the obliquity of the 
shooting. An FEI transmitter, mounted in a 
rectangular opening provided for it in the 
center of a 4x20-ft flag target is shown in 
Figure 1. The two microphones can be seen on 
the right and left sides of the spherical hous¬ 
ing of the FEI transmitter. When the micro¬ 
phone-pair axis is placed parallel to the direc¬ 
tion of tow, indications as to whether misses 
are leading or lagging the target can be given. 



Figure 1 . Aperiodic FEI transmitter mounted in 
center of 4x20-ft Velon plastic flag target. 











INTRODUCTION AND MILITARY REQUIREMENTS 


41 


The sum of the responses of the two micro¬ 
phones to the ballistic shock wave is known as 
the sum response, and because of the acoustics 
of the spherical encasement the sum response 
turns out to be an index of the miss distance 
independent of the orientation of the micro¬ 
phone-pair axis relative to the bullet trajec¬ 
tory. In the figure one of the two batteries can 
be seen (attached on either side of the flag just 
below the transmitter). The two radio anten¬ 
nas lead upward from the transmitter and the 
string is attached through a spring to one edge 
of the rectangular hole. When the flag unfurls 
as it is streamed this string operates a switch 
so that the batteries are turned on to operate 
the FEI transmitter. 

2X4 The Two Functions of the FEI, Scoring 
and Informing, Contrasted 

From the foregoing, two distinct functions 
of the FEI become rather clearly defined, 
namely, (1) scoring gunners and equipment, 
by which a quantitative statistical measure¬ 
ment rates the subjects on a relative scale of 
marksmanship performance in as statistically 
reliable a way as possible, and (2) informing 
which is of a more qualitative nature, where 
the object is to inform a gunner or his instruc¬ 
tors, with as little delay as possible, as to the 
nature of the errors of fire as they are being 
committed, chiefly with the idea of accelerating 
the training of the individual. This information 
may, for instance, consist in telling an aerial 
gunner, after each burst of machine-gun fire, 
whether his shots fell to his right or left of the 
target and in what order such misses occurred. 
If a very close round was indicated by the FEI 
(within some roughly specified bull’s-eye dis¬ 
tance), then this information may also be fur¬ 
nished and the round number indicated. By 
such means the gunner can be expected to cor¬ 
relate the information as to his errors and as 
to his more successful rounds, with his memory 
of the appearance of the target in his sights 
and the other pertinent circumstances of the 
occasion so as to build up habits of proficiency. 
This will obviously be most effective if a mini¬ 
mum time elapses between the shooting and 
the information. Clearly the informing func¬ 
tion concerns single rounds (or a small burst) 


and is not statistical. It requires little more than 
qualitative accuracy in the device. Qualitative 
unreliability regarding single rounds, however, 
would be a rather serious source of confusion 
to the trainee gunner. 

The scoring function, on the other hand, re¬ 
quires a device which will define in all cases a 
target of nearly the same size so that scores 
made by different gunners and equipment with 
different FEI’s will be comparable. Here the 
radial size of the effective target is the element 
of chief interest and statistical reliability is 
required rather than qualitative single-shot re¬ 
liability. 

In the present FEI the scoring of gunners 
has been identified with nearly circular con¬ 
centric zones of sensitivity around the target 
center within each of which the passage of a 
bullet is recorded. Thus, the ability to indicate 
or measure radial miss distances independent 
of direction has come to be identified with the 
scoring function. This, however, is partly a re¬ 
sult of the physical nature of the FEI as de¬ 
veloped and is not to be considered as a pri¬ 
mary definition of scoring. On the other hand, 
the qualitative indication of the directionality 
of the errors of fire (e.g., whether the miss is 
fore or aft of the target if the towing is trans¬ 
verse, or is to the gunner’s right or to his left, 
above or below the target, etc., as the par¬ 
ticular application may require) has come to 
be associated with the informing function, 
though here again this is not its primary def¬ 
inition. The qualitative round-to-round indica¬ 
tions of close hits (within some specified radial 
distance) is clearly also part of the informing 
function. 

This function has been chiefly emphasized in 
the training of aerial gunners shooting from 
plane to plane because here the training con¬ 
centrates on individuals rather than on groups, 
and the information can therefore be most ef¬ 
fectively used to accelerate learning. Because 
of the many things which a gunner must think 
about while shooting, it has been generally 
agreed that the instantaneous information sup¬ 
plied to him by the FEI must be of a very 
simple qualitative nature. The indication of a 
close hit and of one of two opposite directions 
of miss seems to be about as much as a gunner 




42 


THE ACOUSTIC FIRING ERROR INDICATOR 


could be expected to profit by in view of the 
complicated and distracting nature of his ac¬ 
tivity. 

To summarize then, the scoring function of 
the FEI calls for a statistically significant and 
quantitative means of measuring and indicat¬ 
ing the number of rounds which fall within 
concentric circular zones around the airborne 
target center, so that gunners and equipment 
can be compared as to marksmanship results. 
The informing function of the FEI calls for 
qualitatively correct indications to gunners or 
their instructors, from round to round, as to 
the nature of their errors of fire while they are 
being committed or immediately thereafter. 
Scoring is statistical, and aims at rating 
marksmanship. Informing is nonstatistical and 
aims at training marksmen. 

2.3.5 Different Phases of the Problem 
Arising from Various Service Interests 

The Service interests in the FEI have been 
chiefly the following: Antiaircraft Artillery, the 
Air Forces, and the Navy (ship-to-plane fire 
and plane-to-plane rocket fire). The Air Force 
interests may be subdivided into fixed gun¬ 
nery (the training of fighter pilots) and flex¬ 
ible gunnery (the training of waist or turret 
gunners shooting from bombers at attacking 
fighter planes). 

Antiaircraft Artillery 

The Antiaircraft Artillery [AAA] represents 
the interest of the Army Ground Forces which 
was the first and most active interest exhibited 
on the part of the Services. We list below vari¬ 
ous requirements and conditions imposed by 
the AAA needs. 

Types of targets requested. Towed sleeves, 
flags, and gliders and OQ and PQ radio-con¬ 
trolled model planes. The use of towed sleeves 
was largely abandoned before the final form of 
the FEI was developed which was primarily 
for use in towed flags of Velon fabric e and in 
a Navy type of towed glider, the 16-ft winged 

e Much work was done in an effort to use metal wire 
mesh flags, the metal cloth itself being used as the radio 
antenna, but with rather unsatisfactory results. The 
Velon material makes a more durable target and its in¬ 
sulating qualities permit the use of straight wire an¬ 
tennas threaded into the cloth with which very satisfac¬ 
tory radio transmission is obtained. 


target Mark I Model 1. Formal Service request 
for the OQ and PQ model plane application 
originated too late to permit completion of this 
development. It was definitely established in 
tests, however, that the noise of the motor and 
propeller on the model planes did not interfere 
with the operation of the FEI at the shock- 
wave amplitudes for which it is designed. 

Towing speeds. From 125 to 250 mph. A 
great deal of towing is done at present just 
below 200 mph. 

Calibers. Caliber .50 and 40 mm were first re¬ 
quested and the FEI has been developed for 
these. Interest in larger calibers and much 
higher altitudes (for the targets) has been ex¬ 
pressed but time has not permitted this de¬ 
velopment. 

Target scoring zone sizes. At the outset it 
was very difficult to find representative magni¬ 
tudes of gunners’ radial miss distances. Choice 
of scoring-zone radii had to be made from 
rather rough guesses. The final choices of FEI 
zone dimensions, made after much experience 
with the FEI in actual shooting, have been 
compromises between the best size of target 
from the statistical point of view and the need 
to stay sufficiently close to the transmitter to 
keep the shock-wave intensities high in com¬ 
parison to accidental disturbances, noise, etc. 
Zone boundaries are defined at three different 
radii. For .50 caliber the zone radii are 2 y 2 yd, 
5 yd, and 10 yd. For 40 mm the radii are 4, 15, 
and 25 yd. 

Types of course. In antiaircraft target prac¬ 
tice the guns are usually located at intervals 
along a firing line. Transverse courses are then 
towed parallel to this line at perhaps 1,000 to 
1,500 yd slant range above the guns. The guns 
fire in succession as the target passes by, each 
gun ranging over a horizontal angle of perhaps 
±45 degrees either side of the perpendicular 
to the course. Incoming courses are also 
though less frequently used in which the plane 
tows the target at right angles to the firing 
line, passing nearly directly over some of the 
guns. Random, irregular, or unexpected cours¬ 
es have rarely been used. 

Power supply. 110-v 60-c alternating cur¬ 
rent from battery M.G. sets driven by gasoline 
engines. 





INTRODUCTION AND MILITARY REQUIREMENTS 


43 


Aerial Gunnery (Flexible Gunnery, 
Bomber-to-Fighter Fire) 

The training here is concentrated upon indi¬ 
vidual gunners and the general emphasis on 
informing is greater since the learning problem 
is a very difficult one because of the motion of 
both firing plane and target. The rapid changes 
in tactics with the changing needs of the war 
made it difficult to keep both FEI and target 
development for aerial gunnery abreast of new 
requirements. For example, at the outset, 
training on targets such as sleeves or flags 
flown parallel to the shooting plane were con¬ 
templated but later this was replaced by “pur¬ 
suit-curve” courses in which the target ex¬ 
ecutes a course more or less simulating the 
curve of pursuit which an attacking fighter 
must fly in order to keep his own fire directed 
at the bomber. Still later the use of large 
bomber formations (in which flexible gunners 
were required to shoot at fighters not neces¬ 
sarily attacking their own planes) tended to 
render the pursuit-curve training course ob¬ 
solete and to necessitate more random types of 
course for the targets. The adoption of pur¬ 
suit-curve courses led to the abandonment of 
flag targets since the course is directed near¬ 
ly head-on along the gunner’s line of sight and 
the flag, seen on edge, is nearly invisible. For 
this reason the Navy towed glider (16-ft winged 
target) was adopted and certain features of 
the FEI had to be somewhat redeveloped for 
this new target. These shifting trends have 
modified requirements as to the type of in¬ 
forming desired. Starting with lead-lag in¬ 
forming (telling whether shots are fore or 
aft) the emphasis shifted to right-left inform¬ 
ing and then to above-below informing for the 
pursuit-curve courses. It is difficult, if not im¬ 
possible, with the FEI in its present state of 
development, to adapt it to informing as to 
directionality of miss for a random, unpredict¬ 
ed course, although informing as to close hits 
and scoring, for reasons which we shall see 
later, can still be furnished satisfactorily for 
such random courses. A list of requirements 

f Gliders must have conductive coatings beneath the 
fabric cover removed. These coatings, usually supplied 
to give radar reflection, interfere with the effectiveness 
of radio transmission of FEI signals. 


imposed by more recent flexible gunnery needs 
is listed below. 

Types of target requested. Towed gliders f 
for pursuit-curve training were requested, with 
an FEI transmitter mounted on projecting 
struts in front center and with gimbal mount¬ 
ing permitting different orientations of the 
microphone axis. 

In Figure 2 an FEI transmitter is shown in 
the gimbal mounting on the supporting struts 
which project forward from the nose of the 



Figure 2. Aperiodic FEI transmitter mounted in towed 
glider target. 

fuselage and carry the batteries. By moving 
bolt holes in the gimbal mounting a wide choice 
of orientations for the microphone-pair axis is 
available. The antenna wires extend laterally 
from either side of the FEI transmitter to the 
forward projecting tips of struts mounted on 
either wing tip. These struts should be long 
enough to support the antenna wires nearly 
perpendicular to the axis of the glider in order 
to obtain a sufficient range of angles over which 
transmission is strong for both radio channels. 
Towing speeds. From 125 to 250 mph. 
Calibers. Caliber .50 only. 

Rate of machine-gun fire. Up to 20 rounds 
per second. The present FEI can furnish and 
record separate data on each and every round 
at this rate but no faster. Signals occurring 
at closer intervals than 1/20 second interfere 
in the electrical circuits of the receiver. In 
turret fire, therefore, with two guns firing in 
random relation to each other, this is likely to 
occur. Hence use of only one gun is recom- 







44 


THE ACOUSTIC FIRING ERROR INDICATOR 


mended, or else the guns must be synchronized 
and timed to permit no two rounds closer than 
1/20 second. By somewhat more elaborate cir¬ 
cuits in the receiver this minimum resolving 
time could probably be diminished tenfold, but 
time has not permitted this improvement. 

Target scoring zone sizes. Zones of radii 
2 y 2 , 5, and 10 yd. 

Types of course. Pursuit curve and random 
courses. 

Location of receiving station. In B-29 or B-17 
bomber. 

Power supply. Primary supply 24-v direct 
current. This necessitates use of an inverter 
to give 110-v alternating current in order that 
the same design of receiving station can be 
used as the receiver for the Ground Forces. A 
standard inverter which has worked success¬ 
fully for this has 0.75-kva output at 400 c, 115 
v (normally used for power supply to auto¬ 
matic pilot). 

Fixed Aerial Gunnery 

The requirements in fixed gunnery are very 
ill-defined because no actual use of the FEI has 
been made by this Service branch to date. 
Interest is still shown by them however. The 
fixed gunnery application of the FEI differs 
from the flexible one in that the course flown 
by the target is straight but the angle of the 
bullet trajectories with that course varies over 
a very wide range. Thus the “aspect angle” 
as it is called, namely the complement of the 
angle between the bullet trajectory and the 
axis of microphone pair in the FEI transmitter, 
varies over a very wide range and, for reasons 
which will be more evident later, this is likely 
to render unreliable the indications of the di¬ 
rectionality of firing errors for at least part 
of the time. Scoring, however, and informing 
as to proximity of hits remain reliable. Another 
way in which fixed aerial gunnery probably 
differs from other FEI applications is that the 
receiving station cannot conveniently be situ¬ 
ated in the gunner’s plane both because of 
space limitations in the fighter plane, and be¬ 
cause a separate operator for the FEI is de¬ 
sirable, and fighter planes are frequently one- 
man machines. The proposed solution is to 
situate the FEI receiving station in the towing 


plane from which the FEI operator relays the 
information regarding the gunner’s errors to 
the gunner over the regular radio communica¬ 
tion set after each pass or burst of machine-gun 
fire. 

As regards towing speeds, targets, calibers, 
and power supply, the fixed aerial gunnery 
requirements are probably identical with those 
listed for flexible aerial gunnery. 

Navy 

At the outset, the Navy interest in the FEI 
was indicated solely for target practice in ship- 
to-plane fire. The proposal to use it in training 
with plane-to-plane rocket fire did not arise 
until 1944, and, as suitably fast and accurate 
plane-to-plane rockets had not as yet been de¬ 
veloped, realization of this purpose has been 
necessarily retarded. Existing rockets for 
plane-to-plane shooting only barely reach prac¬ 
tical shock-wave velocities if fired forward 
from an already rapidly moving plane. As 
very little is known about ballistic shock waves 
from rockets, experimental work using static 
firing with the FEI was carried out. The FEI 
transmitter was mounted on wires between 
poles so that shots could be fired at known miss 
distances from it. It was also necessary to 
provide an elaborate installation with rocket- 
propelled launching carriages operating on 
350-ft rails, to give the rockets the required 
initial velocity. These facilities were under 
construction by the Navy at the Naval Ord¬ 
nance Testing Station at Inyokern, California, 
but because of the pressure there of higher 
priority work the facilities were not completed 
by the termination of the NDRC contract. Con¬ 
tinuation of the work by Navy personnel is 
contemplated. 

In ship-to-plane fire a large number of guns 
fire from the very concentrated area of the 
ship’s deck at a plane which is usually ap¬ 
proaching directly toward the guns because 
the ship itself is usually the target. Hence, for 
simulated ship-to-plane practice the courses 
flown by the tow plane are almost entirely 
those in which the target passes nearly directly 
over the gun. Because of the great concentra¬ 
tion of guns used no great effort was directed 
at the problem of training or rating individuals 




BALLISTIC SHOCK WAVES. APPLICATION TO FEI 


45 


or single gun crews in marksmanship. The 
training stations were usually situated on a 
shore with many guns concentrated in a small 
area simulating the deck of a ship. The tar¬ 
gets, usually sleeves or flags, were towed to¬ 
ward the land, so that the shooting was over 
the water. Clearly under such conditions the 
applicability of scoring, save for the entire 
group, is out of the question since there is no 
way of discriminating between the shots. Hits 
within a few yards of the target were shown 
by trial with the FEI to be so rare that the 
above-mentioned resolving time of the device 
(1/20 second) was sufficiently short (to avoid 
confusing two hits as one) but no way exists 
to indicate which gun should be credited with 
the close hit. 

The above reasons, coupled with unsatisfac¬ 
tory performance of some of the earlier models 
of FEI transmitter, resulted in a less active 
Navy interest in the device than was the case 
with other Service branches. On the other hand 
the application to plane-to-plane rocket fire, as 
already stated, got under way so late that there 
has been little opportunity for its development 
under this contract. 

24 RESEARCH ON BALLISTIC SHOCK WAVES 
AND THEIR APPLICATION TO THE FEI 

Before passing to a description of the FEI 
and its results, some space must be devoted to 
the nature of ballistic shock waves, acoustic- 
response patterns of the transmitter, the types 
of field tests required, etc. This offers the best 
opportunity to build up a set of basic concepts 
and terminology which will be used henceforth 
throughout the report. Another purpose is to 
give the motivating reasons for the designs 
finally developed. 

2-41 Ballistic Shock Waves, Their Nature 
and Laws of Propagation 

After a considerable amount of purely em¬ 
pirical work on two early forms of the FEI 
(see Section 2.7), an extensive program of 
pure research work on ballistic shock waves 
was necessary in order to explain the surpris¬ 
ing results observed and to permit a more in¬ 
telligent approach to a satisfactory design. We 


give in the following section a brief review 
of well-known pertinent facts together with 
some of the results of this pure research. 

General Description of Shock Waves 

The ballistic shock wave from a bullet is an 
intense acoustic phenomenon occurring only in 
the case of bullets whose speeds relative to 
the surrounding air exceed the velocity of 
sound. It is analogous to the V-shaped head 
and stern waves which occur when a ship is 
propelled rapidly through water. 

It is possible to photograph shock waves di¬ 
rectly and by this means they have been shown 
to be a right circular cone traveling forward 
with the speed of the bullet (see Figure 3). 



Figure 3. Geometry of shock-wave formation. 

The bullet plows aside the air which then 
streams back to fill up the void. In the region of 
the nose, therefore, there is set up a wave of 
condensation which radiates laterally from the 
trajectory while from the tail there is set up 
a wave of rarefaction which radiates similarly. 
The propagation of these two disturbances takes 
place at a velocity approximately that of or¬ 
dinary sound in a direction normal to the 
conical shock-wave front. (Actually, the head 
wave is propagated slightly faster than sound 
while the tail wave is slightly slower.) Sup¬ 
pose that the full lines in Figure 3 represent 
the position of the bullet with its accompany¬ 
ing shock wave 1 second later than their posi¬ 
tions as shown by the dotted lines. In the 1- 
second interval while the nose of the bullet 
progressed through a distance v from O to O', 










46 


THE ACOUSTIC FIRING ERROR INDICATOR 


the pressure disturbance traveled a lesser dis¬ 
tance, S, from O to p. The distance v is nu¬ 
merically equal to the velocity the bullet (e.g., 
in ft per second) while S is numerically equal 
to the velocity of the acoustic shock wave in 
the same units. Thus the angle a, the semi¬ 
apex angle of the shock-wave cone, is more 
acute the faster the speed of the bullet. It can 
be computed from the approximate formula, 

sina = -. (1) 

v 

Source and Propagation of 
Shock-Wave Energy 

The source of the shock-wave cone is ob¬ 
viously the trajectory of the bullet. As illus¬ 
trated in Figure 4, each foot of trajectory (e.g., 

SHOCK WAVE FROM SHOCK WAVE FROM 



Figure 4. Energy transfer from trajectory to shock 
wave. The case for a bullet whose velocity is only 
slightly above sonic velocity (full lines) is compared 
with that for a bullet with higher velocity (dotted 
lines). In the case of the low-velocity bullet, the flow 
of shock-wave energy from one foot of trajectory is 
indicated with arrows. At the instant shown, the shock- 
wave energy is in the volume of revolution swept out 
by rotating the shaded areas about the trajectory 
as axis. 

OQ) may be thought of as contributing a sec¬ 
tion of the shock wave, namely the volume 
which would be swept out by the shaded area 
ABCD if the latter were rotated around the 
axis OQ. The source of the very appreciable 
acoustic energy propagated outward in this ex¬ 
panding conical volume is the kinetic energy 
lost by the bullet in traversing the segment of 
its trajectory OQ, and indeed it is probable that 
a major fraction of the force exerted by the 
bullet in overcoming so-called air resistance, 
performs the work of forming the shock wave. 


The small remainder of the work done by the 
bullet against air resistance is converted di¬ 
rectly into heat and, to a small extent, into 
forward-streaming velocity of the air. 

Shock waves are not formed by bullets whose 
velocities are below the velocity of sound. In 
such cases the energy lost by the bullet is still 
given to the air in much the same way as be¬ 
fore but no sharply defined conical front is 
formed. 8 At bullet velocities only slightly 
higher than sonic the semi-apex angle of the 
shock-wave cone is nearly 90 degrees. It is 
valuable to point out in this connection that 
for such a case the source of the energy which 
forms the shock wave may be a part of the 
trajectory at a considerable distance to the 
rear of the bullet. The geometry of this situ¬ 
ation is shown in Figure 4 by the full lines. 
It is clear also from this figure that the energy 
converted into shock-wave energy from the 
segment OQ is, even at the same miss distance, 
concentrated in a smaller volume of space than 
is the case for higher-velocity bullets, the latter 
case being shown by the dotted lines in Figure 
4. It is probable that this in part explains the 
observed fact that the shock-wave intensities do 
not diminish greatly with diminishing bullet 
velocity until the latter comes very close to 
the velocity of sound. 

Wave Forms of Shock Waves 

The wave forms of shock waves from bullets 
of calibers .30, .50, 20 mm, and 40 mm were 
carefully investigated during this work. Two 
types of a very high-speed microphone were 
used. These microphones controlled the y -axis 
deflection of a cathode-ray oscilloscope whose 
£-axis motion was furnished with a “single¬ 
sweep” motion. This single-sweep motion was 
triggered by the shock wave itself through the 
agency of a second microphone placed so as to 

s If, at a specified instant, spheres of radii st„ sU, st 3 , 
etc., are drawn around points of the trajectory occupied 
by the bullet U, t 2 , U, etc., seconds earlier to indicate how 
far sonic disturbances, originating at each of these posi¬ 
tions and instants of time, would have been propagated, 
then it is easy to see that for bullets whose speeds are 
greater than sonic these spheres intersect each other to 
form a sharply defined conical envelope which is the 
shock-wave cone; whereas for bullets whose speeds are 
less than sonic none of the spheres intersect each other, 
each being completely enclosed by its predecessor, so that 
no sharply defined wave front is formed. 









BALLISTIC SHOCK WAVES. APPLICATION TO FEI 


47 


receive the shock wave a little earlier than the re¬ 
cording microphone. Although the wave forms 
obtained from such studies have many smaller 
complex irregularities, certain salient features 
of similarity can be clearly distinguished in the 
case of all the calibers studied. These are now 
discussed because they are important for an un¬ 
derstanding of the action of the acoustic FEI. 
Figure 5 shows with some idealization how the 

H 



Figure 5. N-shaped shock-wave profile. Acoustic 
pressure at fixed point in air plotted as function of time. 


pressure on a stationary microphone diaphragm 
varies with time as the shock wave passes. This 
same profile therefore can also be understood to 
represent a cross section of the pressures exist¬ 
ing in space throughout the thickness of the 
shock-wave cone at a given instant of time 
(with, of course, a reversal of sense and a 
change of scale on the abscissa). There are two 
very abrupt discontinuities of pressure, the 
head and tail (shown at H and T) and the 
energy of the shock-wave disturbance is local¬ 
ized between them. In the photographs of shock 
waves these discontinuities or fronts appear as 
the two very sharp conical boundary lines. 
These lines are never exactly parallel, but di¬ 
verge slightly with increasing distance from 
the bullet. The H front is a sudden rise above 
atmospheric pressure set up by the sidewise 
thrust of the bullet. The pressure then declines 
more or less linearly to a value equally far be¬ 
low atmospheric, and then returns very ab¬ 
ruptly to normal, (the tail discontinuity T). 
The inrush of air at the tail of the bullet is 
caused by the atmospheric pressure trying to 
fill the void created by the transit. The abrupt 
nature of the tail discontinuity does not develop 
until after the shock wave has been propagated 
a foot or so from the trajectory. The process 
whereby this occurs is an important one for 
the understanding of finite sound waves of 
this type, and is explained below. 


Shock-Wave Discontinuities or Fronts ; 
Their Formation and Propagation 

When the pressures and condensations in 
sound waves become appreciable fractions of 
atmospheric pressure and density, it is a well- 
known fact that the portions of the wave above 
atmospheric pressure have velocities slightly 
in excess of the ordinary velocity of sound, 
whereas the portions below atmospheric pres¬ 
sure have velocities slightly less. The reason 
for this is clear. In the portions of the wave 
where the pressure is above atmospheric there 
is a particle velocity in the forward direction, 
and the velocity of sound propagation relative 
to this moving air is, if anything, slightly 
higher than in other parts of the wave be¬ 
cause of the higher temperatures in the high- 
pressure regions. This velocity of sound is 
therefore added to the local particle velocity to 
give the velocity at which the elevated portions 
of the wave form in such a region are propa¬ 
gated. Similar reasoning shows that the nega¬ 
tive portions of the wave form are propagated 
below sonic velocity. There is thus a differen¬ 
tial in the velocity of propagation of the var¬ 
ious parts of the wave profile so that in the 
regions of positive acoustic pressure, the higher 
portions “catch up” on the portions near at¬ 
mospheric pressure, while in the negative re¬ 
gions the lower portions of the wave form lag 
behind those nearer to atmospheric pressure. 
As this process progresses, the rising portions 
of the wave form become steeper and steeper 
(and the falling portions, less and less steep) 
until the wave form eventually becomes vertical, 
or nearly so, at some point. 

The wave form or pressure profile can obviously not 
pass beyond the vertical, for if it did this would imply 
two pressures at the same point in the air at one time. In 
fact, the so-called discontinuity never becomes completely 
abrupt although its thickness may become excessively 
small for large pressure transitions. The thickness is 
limited by two effects, viscosity and thermal conductivity, 
which counteract the steepening tendencies, previously 
described, by transferring heat and momentum across the 
wave front when a certain limiting abruptness has been 
reached. For weak shock waves (pressure steps of the 
order of a few per cent of one atmosphere) the thickness 
of the front is inversely proportional to the amplitude of 
the pressure step. See Table 1 farther on in this section 
for representative front thicknesses which are of the 
order of fractions of a millimeter for the ballistic shock 
waves with which we are concerned. 








48 


THE ACOUSTIC FIRING ERROR INDICATOR 


As other portions of the wave form catch up 
with this discontinuity they coalesce with it 
and add to its height. Crossing over this front 
by other portions of the wave profile never 
occurs. Each new and higher segment of the 
wave form that arrives at the discontinuity 
merely makes the discontinuity higher with a 
consequent slight increase in its velocity of 
propagation. Similar statements may be made 
regarding the building up of the tail wave. 
Each new and lower segment of the wave form 
lags behind until it reaches the discontinuity 
and it then merely increases the amplitude of 
the tail discontinuity with a consequent slight 
decrease in its velocity of propagation (relative 
to the undisturbed air mass). Figure 6 is a 



Figure 6. Progressive changes in wave form as ballistic 
shock wave is propagated to left. 


sketch illustrating the progress of such a 
process. 11 This progressive joining of various 
parts of the wave profile to form the two dis¬ 
continuous fronts can also be clearly seen on 
the direct photographs of shock waves. 

The advance of the crests and the retarda¬ 
tion of the troughs in the wave profile have 
an analogy in surface waves on water as they 
advance toward a shore over a sloping beach, 
but in the latter case the wave profile on the 
forward side can pass beyond the vertical so 

h The reader is referred to Appendix IIP for a detailed 
account and a theoretical treatment of the formation of 
the discontinuities. The velocity of propagation of a dis¬ 
continuity is there shown to depend on the amplitude of 
its pressure step according to the Rankine-Hugoniot dis¬ 
continuity relationship. 


that a breaker is formed. In the case of sound 
waves this is impossible since it would imply 
more than one pressure at a given point in 
space. 

The discontinuities just described are the 
most characteristic features of a shock wave. 
They are completely different from any wave 
form occurring in the case of ordinary sounds. 
It is also clear because of the above-mentioned 
sharp discontinuities that there may be present 
in ballistic shock waves, even after they have 
traveled 60 to 80 yd, Fourier components of 
very high frequency. In ordinary sound waves 
such high frequencies are very rapidly ab¬ 
sorbed, but in the case of shock waves, the 
steepness of the fronts is maintained in spite 
of such absorption by the effects just outlined. 

It is clear from a study of Figure 6 that a 
large variety of initial wave forms will, by the 
process just explained, eventually assume ap¬ 
proximately the N-shaped profile. 

Amplitudes Associated with 
Ballistic Shock Waves 

It is possible to obtain a measure of the 
amplitude of the discontinuities of the N wave 
by observing the rate at which the period, T, 
of the N wave (measured between the H and 
T discontinuities) increases with increasing 
miss distance. Because of the difference in 
velocity between the H and T discontinuities, 
there is a progressive change of this period T 
and the spatial separation between the two 
fronts with increasing distance from the tra¬ 
jectory. For the case of 40-mm fire this shock- 
wave period has been found to be • twice as 
long at 80 yd from the trajectory as it is at 
4 yd. 

The experimentally established fact of the differen¬ 
tial in the speeds of propagation of the two discon¬ 
tinuities extending as it does out to such large dis¬ 
tances, together with the characteristic N-shaped wave 
profile and the description of its formation are, it is 
believed, little-known and valuable contributions to the 
knowledge of shock waves made as by-products of the 
research and development on the acoustic FEI. Since 
this work was done attention has been called to the 
work of L. D. Landau. -13 He describes a wave form 
of this type which theoretical considerations led him 
to expect as the eventual profile of a shock wave after 
propagation over large distances. 

Were it not for the doppler effect which 
modifies this period for a moving microphone 
the best way by far for measuring miss dis- 











BALLISTIC SHOCK WAVES. APPLICATION TO FEI 


49 


tance would, no doubt, be by observations of 
this period. The curves of Figure 7 show, on 
logarithmic coordinates, how the period T' of 
the shock wave increases with increasing dis¬ 
tance from the trajectory. These curves were 



MISS DISTANCE IN YARDS, LOGARITHMIC SCALE 

Figure 7. Period of shock wave versus miss distance. 

plotted from measurements of the shock-wave 
period T made on oscillograms. The periods 
for four calibers are shown. It should be noted 
that all the curves have such a slope on loga¬ 
rithmic paper as to indicate that T increases 
approximately as the fourth root of the miss 
distance. 3 

Because of this change in N-wave period 
with miss distance, one of the earlier attempted 
forms of the FEI, in which highly resonant 
diaphragms were used in the microphones, had 
very peculiar acoustic-response characteristics. 
The highly resonant diaphragms of 1,600- and 
2,400-c frequencies were provided, with the 
idea of using a single r-f channel of communi¬ 
cation, and distinguishing between the signals 
from the two microphones in a bilateral FEI 
transmitter by tuning the diaphragms to dif¬ 
ferent audio frequencies. With these resonant 
diaphragms it turned out that at certain miss 
distances an unfavorable ratio between dia¬ 
phragm period and N-wave period actually re¬ 
sulted in complete unresponsiveness of such 
diaphragms, while at other miss distances their 
response was greatly enhanced. In the case of 
40-mm caliber fire, with a 2,400-c resonant dia¬ 
phragm, a null response actually occurs at very 
close miss distances of the order of 1 yd, and 
again at 20 yd, and a maximum of response 
occurs at about 2 y 2 yd and again at 40 yd. Fig¬ 
ure 8 shows such a response curve. Although 


this figure was computed theoretically from a 
solution of the differential equations for the 
motion of the diaphragm, as acted upon by an 
N wave having the periods observed (Figure 
7), it turns out to be very satisfactorily checked 
by experiment. The null points referred to have 
been observed by actual shooting as has also the 
intervening maximum point. These results fur¬ 
nish confirmation of the general picture of the 
ballistic shock-wave propagation process. 


Possible Existence of “After Waves” 


Before leaving the subject of wave forms it 
should be mentioned that there is possible evi¬ 
dence of some acoustic disturbances trailing 
to the rear of the N-shaped wave profile. These 
are only very faintly visible, if at all, on the 
direct photographs of shock waves but are to 
be seen in some of the oscillograms, immedi¬ 
ately following the tail discontinuity. They 
seem to be of a very irregular nature and are 
not very definitely related in phase or wave 
form to the N shape itself. Curiously enough, 


oo 


“S 30 


ox 

t|$ ' 

-I 

>3 

t-d 

£0 


1 

1 

1 

1 

I 

\ 








\ 

\ 

100 YDS/MISS OIST INVERSE FIRS1 

SHOCK WAVE AMPLITUDE DECAY 

r POWER 

LAW OF 



RESPONS 

MICROPF 

»E OF 2 4< 
IONE TO 

50 CPS F 
40mm 

o 

z 

> 

2 

IT 












O IO 20 30 

MISS DISTANCE (YARDS) 


50 60 70 80 

40 mm AMMUNITION 


Figure 8. Theoretically computed response curve 
versus miss distance. 


the process whereby the H and T discontinu¬ 
ities are built up in the N wave does not seem 
to be operative to form similar discontinuities 
in these “after waves.” We are unable to ex¬ 
plain this at present. In fact, the entire nature 
and origin of the after waves is obscure. It 
may even be that they are not really present in 
the air but are in some fashion a parasitic 
effect coming perhaps from vibrations set up 
in the microphones. 

Dependence of Shock-Wave Pressure 
Amplitude on Miss Distance 
The microphone measurements of the peak 
pressure elevations in shock waves as a func- 















































































50 


THE ACOUSTIC FIRING ERROR INDICATOR 


tion of miss distance (the perpendicular dis¬ 
tance from the trajectory measured to the 
point of observation) show that these pressure 
elevations vary inversely as the nth power of 
the miss distance where n has the value 
Since the energy density in the wave is propor¬ 
tional to the square of the pressure elevation 
this implies that the energy density falls off as 
the inverse 1.5 power (or faster) of the miss 
distance. This is a somewhat faster rate of decay 
with distance than would be expected if (1) 
there were no dissipation of shock-wave energy 
into other forms, and (2) the thickness of the 
N-shaped wave form (measured between the 
H and T fronts) were not progressively increas¬ 
ing. Were the two causes not operative the simple 
geometry of shock waves (Figures 3 and 4) 
would give an inverse first power law of de¬ 
pendence of energy density on miss distance 
and an inverse one-half power law for the 
peak excess pressure. The theoretical reason 
for the observed dependence is given in Ap¬ 
pendix III. 3 

Absolute Values of Shock-Wave 
Pressure Amplitudes 

Estimates have been made of the absolute 
values of the peak pressure elevations in shock 
waves, based on the rate of increase of the 
N-wave period T', and using the Rankine- 
Hugoniot discontinuity relationship between 
propagation velocity and pressure elevation. 
These agree as to order of magnitude with 
estimates based on the response of the micro¬ 
phones. For shock waves at 40 yd distance 


from the path of the bullet they indicate, as 
shown in Table 1, very high momentary sound 
intensities of the order of a million times the 
intensity of ordinary speech at 1 yd distance, 
for example. 

In the table, formulas are given in terms of 
8, the peak pressure elevation in the shock 
wave expressed in fractions of the atmospheric 
pressure. 


where pi is the undisturbed atmospheric pres¬ 
sure and p 2 — Vi is the height of the pressure 
step in the two N-wave discontinuities (which 
are assumed to be substantially equal). The 
numerical fractions in each formula are in 
reality functions of the specific heat ratio, y, 
for air. The values used for 8 in computing 
Table 1 are obtained by observing the slope of 
the curve relating T', the N-wave period, with 
miss distance. This method, which we call the 
dT'/dd method, relies on the fact that the ex¬ 
cess and defect in the propagation velocities 
of the H and T discontinuities of the N wave 
are related to their pressure-step amplitudes 
by the Rankine-Hugoniot discontinuity rela¬ 
tionship which for weak discontinuities as¬ 
sumes the simple approximate form 



e is the propagation velocity of the discontin¬ 
uity, and c is the propagation velocity for or¬ 
dinary sounds. 


Table 1. 

Absolute ballistic shock-wave magnitudes for air. 



Formula for 
shock-wave 
magnitude 

40 mm at 40 yd 
miss distance 

.50 cal. at 40 yd 
miss distance 

Ordinary 
speech at 

1 yd 

Peak pressure elevation 
dynes/cm 2 
(lb/in. 2 ) 

P*-Pi 

2200 

(0.032) 

1500 

(0.022) 

2 

(0.00003) 

Max particle velocity 
cm/sec 

5/7 5c 

52 

35 

0.05 

Max temperature 
rise C 

2/7 5 r, 

0.19 

0.13 

0.0002 

Peak intensity 
watts/cm 2 

5/75 2 P,c 

0.0115 

0.0052 

10-8 

Shock-wave discontinuity thickness 
cm 

3X10- 5 5" 1 

0.014 

0.020 













BALLISTIC SHOCK WAVES. APPLICATION TO FEI 


51 


Reproducibility of Transmission 
of Shock-Wave Amplitudes 
Through Outdoor Air 

The reproducibility with which shock-wave 
amplitudes are transmitted through outdoor 
air over large distances, such as 10 to 40 
yd, is higher than experience with ordinary 
sounds might lead one to anticipate. In making 
measurements of the shot-to-shot reproduci¬ 
bility of the response of the FEI, several other 
causes beside the irreproducibility of shock- 
wave transmission are simultaneously opera¬ 
tive and difficult to separate from the latter. 
These are the irreproducibility of bullet veloc¬ 
ity, of miss distance, of transmission of shock 
waves through outdoor air, of FEI-transmitter 
and receiver response, and of the graphs from 
the recording instrument. Clearly, the varia¬ 
tion in transmission can contribute only a frac¬ 
tion of the total observed shot-to-shot vari¬ 
ability of response. The simultaneous opera¬ 
tion of all six of these sources of irreproduci¬ 
bility is some 6 or 7 per cent for the case of a 
transmitter in still air. In towed flight there 
seems indication that the reproducibility is less 
satisfactory probably because of pressure dis¬ 
turbances, turbulence, etc. For the stationary 
transmitter an upper limit of the standard 
deviation arising from the irreproducibility of 
transmission of shock waves through outdoor 
air alone is probably not more than 5 per cent 
of the mean observation and perhaps very 
much smaller than this. These figures, holding 
for distances of propagation of the order of 40 
yd, represent much higher reproducibility than 
is encountered for ordinary infinitesimal 
sounds. The explanation is perhaps to be 
found in the large particle velocities, pressure 
amplitudes, temperature rises, and condensa¬ 
tions associated with shock waves, which per¬ 
haps far exceed the accidental local fluctuations 


of air velocity, pressure, temperature, and 
density encountered in outdoor air. 

Dependence of Shock-Wave Amplitude 
(Peak Value) on Projectile Caliber 

The measurements with calibers .50, 20 mm, 
and 40 mm indicate that the relative peak 
amplitudes of the shock waves from bullets 
of these three different calibers referred to 
the peak amplitude of caliber .50 as unit are 
as given in Table 2, for distances of about 10 
yd from the trajectory. These ratios are appli¬ 
cable to the three calibers in question over a 
range of miss distance from 5 to 20 yd. 


Table 2. Relative shock-wave magnitudes. 


.50 cal 

1.00 

20 mm 

2.0 

40 mm 

3.8 


Dependence of Peak Value of the 
Shock-Wave on Projectile Velocity 

The variation of shock-wave amplitude with 
range from the gun (and therefore with pro¬ 
jectile velocity) is extremely small, so small in 
fact that it is difficult to be certain whether 
the variation is not due to errors of measure¬ 
ment. With increasing range from the gun it 
becomes increasingly difficult to place shots 
correctly as to miss distance and also increas¬ 
ingly difficult to estimate the error of place¬ 
ment with theodolites. For example, at 1,500- 
yd range and for the case of 40 mm for which 
n = 0.9, an error in fire of 1 mil results in a 15 
per cent error in miss distance at a miss dis¬ 
tance of 10 yd, with a consequent error in 
shock-wave amplitude of about 0.9 of this 
amount (13.5 per cent). Such errors are diffi¬ 
cult to avoid. 

Table 3 shows typical data regarding the 
variation of shock-wave amplitude with range 


Table 3. Range variation of sum response of aperiodic microphone transmitter 
to shock waves from three different calibers. 


Range 
from gun 
in yd 

40 mm at 20 yd 
miss distance 
speed fps response 

20 mm at 10 yd 
miss distance 
speed fps response 

.50 cal. at 10 yd 
miss distance 
speed fps response 

250 

2,610 

4.52 

2,245 

2.07 

2,325 

3.01 

500 

2,430 

5.74 

1,790 

2.21 

2,190 

2.81 

1,000 

2,100 

4.61 

1,220 

1.47 

1,725 

2.44 

1,500 

1,800 

4.31 



1,320 

3.84 




























52 


THE ACOUSTIC FIRING ERROR INDICATOR 


from the gun for calibers .50, 20 mm, and 40 
mm. The approximate projectile speeds at 
these ranges are also tabulated. 

Dependence of Shock-Wave Period, T', 
on Range (Projectile Velocity) 

The period T' (between H and T discontinu¬ 
ities of the shock wave) which one might at 
first glance expect to be inversely proportional 
to projectile velocity turns out to be surpris¬ 
ingly insensitive to velocity. It has not been 
possible to establish satisfactory evidence for 
any dependence of this type whatever. In 
Appendix IIP it is shown that an absolute 
relationship between shock-wave amplitude and 
period and miss distance containing no arbi¬ 
trary constants whatever drops out of the 
theory for the case of large miss distances in 
a somewhat surprising way. This relationship 
is satisfied by the experimental observations to 
within the range of their accuracy. It is a 
matter of doubt :! to what extent the position of 
the tail discontinuity and the wavelength of 
the shock wave can be regarded as determined 
by the length of the bullet and to what extent, 
especially at large distances from the tra¬ 
jectory, these must be regarded as determined 
by the laws of propagation. 

2 . 4.2 p ure an( J Developmental Research 
Methods and Equipment 

Field Tests with Static Firing 

A very large amount of field testing known 
as “static firing” has been required for the 
pure research and also the developmental work 
in connection with the FEI. In static firing 
the microphonic element is not towed through 
the air but a detector of suitable design is 
suspended by wires on one or more poles about 
25 ft above the ground on a firing range. Shots 
are fired at measured ranges and miss distances 
from this, either to measure the shock-wave 
amplitude or to ascertain the response patterns 
of an FEI transmitter. It is desirable to define 
a few terms which will be constantly used both 
in connection with static firing and elsewhere. 
The target plane is an imaginary plane passing 
through the FEI transmitter perpendicular to 


the gun target [GT] line (straight line from 
gun to FEI transmitter). 1 

Response patterns of the FEI transmitter are 
plotted in this plane. Curves of iso-response 
can be plotted in this plane, i.e., the loci of the 
piercing points in the plane of bullet trajec¬ 
tories which excite the same response in the 
FEI transmitter. These form a complete sys¬ 
tem of loci descriptive of the response charac¬ 
teristics for one aspect angle or orientation of 
the FEI transmitter relative to the direction 
of shooting. Such response patterns must be 
studied for different aspect angles, for different 
ranges and for different calibers. A great many 
rounds must be fired because, besides the neces¬ 
sity of exploring response as a function of so 
many variables, it is also necessary that each 
shot be duplicated five or six times in order to 
average out statistical fluctuations in shock- 
wave response. Because of inaccuracy in fir¬ 
ing, somewhat more shots must usually be 
placed that can be used. The placement of each 
shot in the target plane is observed with two 
BC scopes (battery command telescopic the¬ 
odolites) which are provided with horizontal 
and vertical scales in the field of view so that 
the error in placement of the shot (right or 
left and in elevation) relative to the desired 
point in the target plane can be recorded. The 
BC scopes are situated, one at the flank and 
one directly behind the gun, preferably on 
towers, the flank scope being used as a check 
on elevation and the rear scope as a check on 
right or left error. The range is provided with 
surveyed markers indicating standard hori¬ 
zontal miss distances in the target plane. The 
method generally employed for supporting the 
FEI transmitter utilizes a specially designed 
light, tubular supporting framework which 
permits orientation of the transmitter in mea¬ 
surable orientations in polar coordinates with¬ 
out offering serious obstacles to the sound 
waves. This frame runs on an oblique funicular 
trolley cable from the ground up to the top of 
the pole by means of a rope and pulleys, the 

' A more strictly accurate definition of the target plane 
should specify it as perpendicular to the trajectory of a 
bullet making a direct hit on the FEI transmitter. The 
difference between these definitions is slight for the close 
ranges used at present but may become important if 
larger calibers and higher altitudes are used. 






BALLISTIC SHOCK WAVES. APPLICATION TO FEI 


53 



Figure 9. Funicular trolley and Lincoln range. 


vertical plane through the oblique cable being rangements on a small-caliber experimental 
parallel to the direction of shooting on the range near Pasadena are shown in Figure 9. 
range. Three photographic views of these ar- At C and D the protractor scales for measur- 



Figure 10. Perspective view of Camp Irwin range. 





































54 


THE ACOUSTIC FIRING ERROR INDICATOR 


ing the polar coordinate angles of orientation 
of the transmitter are shown. A is a clamping 
ring provided for holding the transmitter and, 
B a clamp for the batteries. In one of the views 
the “fox hole” where the measuring instru¬ 
ments and observers are situated is visible. 


preamplifier (to convert the output to low 
impedance) were connected by cable directly 
to a single-sweep oscilloscope located in a safe 
flank position. The sweep was triggered by 
a carbon microphone situated to receive the 
shock wave 2 or 3 msec earlier than the re¬ 



in Figure 10 is shown a perspective view of 
the general layout of the Camp Irwin range 
where the bulk of the research was done, and 
Figure 11 shows the range and launcher as 
planned at the Inyokern Naval Ordnance Test¬ 
ing Station for the special work with rockets. 
At the latter range the transmitter trolley op¬ 
erates on one of a set of horizontal wires 
strung transversely across the range between 
pairs of poles at different distances in front 
of the launching position. 

Equipment for Study of Wave Forms 
In the pure research on shock-wave ampli¬ 
tudes and wave forms other types of micro¬ 
phone such as a piezoelectric quartz crystal 
sound cell were at first used in place of an FEI 
transmitter. In this case the sound cell and 


cording microphone. The oscilloscope had a 
persistent screen and was provided with mo¬ 
mentary spot intensification by high-voltage 
acceleration near the screen during the sweep 
interval. This permitted photography of the 
traces at high writing speeds. 

Quantitative Field Measuring Equipment 
for Shock-Wave Peak Amplitudes 

Most of the data on shock-wave amplitudes 
as a function of caliber, range, miss distance, 
and angular positioning of the FEI transmitter 
were made by means of special field-measuring 
equipment. This operated on the same princi¬ 
ple as that used for the Service application of 
the FEI, save that provision for quantitative 
measurements, rather than mere classification 
of shots into radial miss-distance zones, had to 


GflNFIDEffTfflj 












BALLISTIC SHOCK WAVES. APPLICATION TO FEI 


55 


be made. The recording equipment, usually 
located in a truck at a safe flank location, was 
operated by frequency modulation radio link 
to the FEI transmitter on the pole. The N- 
wave electrical impulses in the frequency mod¬ 
ulation receiver are indicative of the amplitude 
response of the microphone diaphragms. These 
were pulse-lengthened j in several stages so as 
to give slowly decaying pulses that are nearly 
direct current. A decay time constant of 2 
seconds with suitable impedance is satisfactory 
for driving an Esterline-Angus recording mil- 
liammeter of short period (0.75 second for full 
deflection). In this way the deflection of the E.A. 
milliammeter measures the peak value of the 
first rise in the N-wave pulse. Two such mea¬ 
suring systems were provided, one for each 
of the microphone channels in the dual-micro¬ 
phone FEI transmitter. 

The Firing Error Oscillograph 

The electrical N-waves output by the dis¬ 
criminator of the frequency-modulation FEI 
field-measuring receiver were also studied with 
photographic oscillograms by means of a cath¬ 
ode-ray oscilloscope. This method is very 
cumbersome, however, where a great number 
of shock-wave records must be taken unless 
specially designed equipment for the purpose 
is constructed. It has the great advantage 
nevertheless of avoiding the necessity for pulse 
lengthening with its attendant uncertainties 
as to accuracy, and for this reason early in 
1945 work was started on the design and 
construction of a firing error oscillograph 
[FEOp with the idea of using this method 
exclusively for quantitative field measurements 
of FEI response. The oscillographic records 
from each of the two FEI channels are photo¬ 
graphed on moving picture film in a camera 
especially designed for the purpose. The film 
is automatically set in motion by the firing of 
the gun so as to minimize the required amount 
of film when hundreds of rounds must be re- 

j The electrical impulse charges a condenser (shunted 
with a high resistance) through a rectifying diode. The 
voltage on the condenser is the pulse-lengthened output 
and with suitable impedance transformation by a 
cathode-follower stage a second stage of pulse-lengthen¬ 
ing may in its turn be driven from the first. The multi¬ 
stage method is necessary when the time constant of a 
given pulse must be increased by a very large factor. . 


corded. Auxiliary oscilloscopes are also pro¬ 
vided for visual observation as a check in case 
of failure of operation, so that repeat shots 
can be made. The FEO was not used to a great 
extent as it was only completed and ready for 
use late in July 1945. Enough work was done 
with it to prove its value and operability, how¬ 
ever. 

The Firing Error Camera 

The firing error camera [FEC] 35 was de¬ 
signed and constructed in order to furnish a 
method of validation of the FEI reports as to 
shots in the case of actual towed flight. Its 
purpose is to determine photographically the 
actual position of each bullet relative to the 
target at the instant when that bullet reached 
the target plane. k 

A deVry 35-mm-film camera is used with the 
film running at 10 frames per second driven 
by a synchronous motor. At a point in the 
mechanism near the driving sprocket, where 
the motion of the film is not intermittent but 
continuous, there are projected on the edge of 
the film the images of three small slits behind 
each of which is situated a neon light. Each 
of these three lights can be very briefly but 
intensely illuminated by a condenser discharge. 
One of them is to be triggered by a microphone 
circuit operated by the muzzle blast of the gun. 
A second is lighted in a similar way when the 
shock-wave signal, arriving at the FEI re¬ 
ceiving station, announces that the bullet has 
passed the target. The purpose of the third 
light is to mark the instant at which the center 
of the camera shutter opening coincides with 
the center of the lens aperture. The shutter 
opening is made adjustable and in use is re¬ 
duced to the smallest arc permitting adequate 
exposure. 

The time interval between muzzle blast and 
shock-wave signal gives the time of flight of 
the bullet and from this the slant range can 
be obtained with the aid of the firing tables. 
At the same time the camera, provided with 

k The idea of a shock-wave triggered camera for study¬ 
ing firing errors was proposed, and a camera whose 
shutter was timed by the shock wave was constructed in 
Section T of NDRC. The present camera merely indexes 
the instant of the shock-wave signal on the edge of a 
motion picture film on which the tracer bullets are being 
photographed, but the general idea is the same. 








THE ACOUSTIC FIRING ERROR INDICATOR 


56 

a red filter, photographs the tracer bullets and 
the target, and measurements of their separa¬ 
tion on the film can be reduced to actual yards 
since the slant range is known. The rate of 
fire must be restricted during such tests so that 
only one tracer will be visible in the field of 
the camera at a time. Provision of a wide field 
finder telescope attached to the camera is to 



facilitate tracking the target by hand so that 
its image shall remain near the center of the 
field. When the developed film is examined, it 
becomes possible to ascertain, from measure¬ 
ments of the exact position on the film of the 
image formed by the shock-wave triggered 
neon light, the precise instant when the shock- 
wave signal impinged on the FEI transmitter. 
This will in general fall at some instant inter¬ 
mediate in time between the exposures of two 
adjacent frames on the film. The relative posi¬ 
tion of tracer bullet and target will be slightly 
different in these two frames because of the 
relative motion of towed target and bullet pro¬ 
jected on the picture plane. The relative posi¬ 
tion of these two objects at the instant of the 
shock-wave signal can be obtained from the 


above described data by a linear interpolation. 
It is then necessary to make a small correction 
because the shock-wave signal is slightly later 
than the instant when the bullet pierced the 
target plane, which is the instant at which the 
magnitude of the miss is sought. This correc¬ 
tion is called the delay error. The data on the 
film itself, reduced to yards, plus the known 
velocity of the bullets, furnish all that is neces¬ 
sary to make this correction. The corrected 
relative position of bullet and target expressed 
in yards will then permit that round to be classi¬ 
fied as inside (or outside) a specified bull’s-eye 
region and this result can then be compared 
with the report given by the FEI receiving 
station. Figure 12 shows the shock-wave index¬ 
ing camera. 

The FEC suffers from the objection that if, 
as sometimes occurs, the FEI fails to report a 
shock-wave signal for a given bullet, that round 
cannot be located by the FEC. Only rounds on 
which the FEI furnishes reports can be located 
by the FEC for comparison. This implies a 
systematic selection of the data which may be 
too favorable to the FEI and in consequence 
the final validation of the FEI in towed flight 
was made to depend on the Stibitz dual photo¬ 
graphic theodolite method rather than on the 
FEC. 

Flight Tests for Noise Study 
and Mechanical Reliability 

Besides the above-mentioned static firing 
tests, flight tests had frequently to be arranged 
in which the FEI transmitters were towed in 
flags or other targets, with or without shooting 
by field artillery. The purpose of such tests was 
to study chiefly effects of noise and other dis¬ 
turbances in towed flight and also to serve as 
checks on the mechanical operability and relia¬ 
bility of both FEI receivers and transmitters. 
Locations of Field Tests 

Much of the field testing as well as the pure 
research on the FEI development was conducted 
at Camp Irwin, about 30 miles north of Barstow, 
California. This was the nearest practicable lo¬ 
cation where Army ordnance could be made 
available. The large distance coupled with the 
difficulties of transportation and of arranging 
for cooperation with several branches of the 



BALLISTIC SHOCK WAVES. APPLICATION TO FEI 


57 


Services at once (Artillery for shooting, Air 
Forces for towing, etc.) constituted one of the 
most difficult and time-consuming features of 
the work. Towing facilities were usually fur¬ 
nished by March Field near Riverside, Cali¬ 
fornia. It was usual practice to have one repre¬ 
sentative of the contract in the tow plane to 
communicate by radio with the others on the 
ground at the receiving station. 

Only very small (caliber .30) experimental 
firing could be done at a small range situated 
in the outskirts of Pasadena, a few miles from 
the California Institute of Technology. No 
quantitative results of importance could there¬ 
fore be obtained in this location, only qualita¬ 
tive checks of operability. 

A considerable amount of testing work and 
demonstrations to the Armed Forces had to be 
carried out at very distant points such as Camp 
Davis, N. C. (for the Antiaircraft Artillery dur¬ 
ing the earlier phases of the work); Dam Neck, 
Va. (for the Navy); Ft. Meyers, Fla. (for the 
Air Forces during the earlier phases). Later a 
representative of the CIT contract was main¬ 
tained over a period of several months at 
Laredo, Texas, to organize tests for the Air 
Forces. Representatives also spent an aggre¬ 
gate of several weeks at Ft. Bliss, Texas, where 
the camera validation tests of the FEI scoring 
were held. 

25 SUMMARY OF DEVELOPMENT 

2,51 General Design Considerations Imposed 
by Physical and Military Requirements 

The Telemetering Problem 

The acoustic FEI must be situated in the air¬ 
borne target and must transmit its information 
regarding the intensity of the shock-wave sig¬ 
nals, and hence the proximity of the shot, to a 
location near the gunner or the instructor. In 
order to do this with a radio link, the variations 
in the intensity of the received radio signals 
(due to varying transmission distance and other 
conditions) must in no way affect the quantita¬ 
tive significance of the information regarding 
the bullets. This problem in telemetering was 
solved by the use of frequency modulation. The 
earliest type of FEI used in successful field tests 
with towed targets consisted of a single conden¬ 


ser microphone connected with its variable 
capacity as part of the tank circuit of a small 
one-tube r-f transmitter. An f-m receiver placed 
near the gun detected the radio signals and con¬ 
verted the FM into brief audio-voltage pulses 
proportional to the response of the microphone. 
These pulses were suitably amplified and length¬ 
ened to give (1) audible signals in earphones, (2) 
visual indications of various sorts, and (3) per¬ 
manent records on recording meters. Since it is 
the r-f shift or excursion from the carrier value 
which is used to indicate the shock-wave ampli¬ 
tude and since, in the f-m receiver of the FEI, 
the amplitude of the r-f signals is maintained 
constant by a “limiter” for a wide range of 
input-signal levels, the telemetered measure¬ 
ment is kept independent of transmission con¬ 
ditions so long as the signal level is sufficient 
to saturate the limiter. 

Desirability of a Dual-Microphone System 

The idea of providing two microphones in 
the FEI was first suggested as a means of fur¬ 
nishing directional indications regarding the 
misses. A much more important reason for the 
dual system has arisen as the work progressed. 
The use of two microphones with their dia¬ 
phragms at diametrically opposite points of the 
spherical case can be shown, both by theory 
and practice, to give a sum response which is 
the sum of the peak responses of the two micro¬ 
phones and which is practically independent 
of the orientation of the microphone-pair axis 
to the direction of the shock wave. This prop¬ 
erty is of the greatest importance for the scor¬ 
ing function of the FEI for it makes the sum 
response of the FEI independent of the aspect 
angle. The shock wave does not impress its 
N-shaped impulse on the two microphones si¬ 
multaneously and to add the peak amplitudes 
a rather elaborate system of pulse lengthening 
is required. This system is preferably located at 
the FEI receiving station rather than in the 
FEI transmitter, since it is desirable to keep 
the transmitters simple, compact, and light. 
For this reason, as well as for the advantage of 
directional indication, it is highly desirable to 
transmit separately the responses of the two mi¬ 
crophones to the FEI receiving station. Another 
reason for the importance of a dual-channel sys- 



58 


THE ACOUSTIC FIRING ERROR INDICATOR 


tem has come to light rather late in the develop¬ 
ment. This is the discovery that the delay error 
can be very effectively compensated by intro¬ 
ducing a specific gain unbalance in the two 
channels of the receiver, provided the micro¬ 
phone axis in the transmitter is parallel to the 
direction of tow. (See below and Appendix II. 2 ) 

The Resonant FEI and the Aperiodic FEI 

The original thought was to use a microphone 
in which the diaphragm deflections would fol¬ 
low the changes in pressure with some approach 
to fidelity in time. This implies a diaphragm 
of rather high natural period with sufficient 
damping to give a flat frequency response over 
the entire range necessary for an approximately 
faithful delineation of the transient N wave. 
At the outset, however, little or nothing was 
known about the forms of shock waves, and 
the high frequencies associated with the abrupt 
discontinuities even at very large miss distances 
were not suspected. The shock wave was postu¬ 
lated to be a simple pulse of pressure elevation 
having about the transit time of the bullet (of 
order 1 millisecond). 

Early in this empirical stage of the work the 
idea of utilizing microphones with highly res¬ 
onant tuned diaphragms was proposed. The 
thought was to provide two or more such micro¬ 
phones modulating one and the same r-f trans¬ 
mitter so that by tuning the diaphragms to 
different audio-frequency notes the signals from 
the different microphones could be separately 
distinguished by means of electric filters at the 
receiving station. By installing two such micro¬ 
phones in a box facing in opposite directions 
behind appropriate port holes, it was hoped that 
it would be possible to determine the side of the 
box on which a bullet had passed, by knowing 
which of the two microphones had the greater 
response. The shock wave was to excite the 
microphone diaphragm just as the blow of a 
hammer upon a bell or a tuning fork excites 
sustained vibrations in these instruments. It 
was supposed that this exciting impulse would 
be short compared to one cycle of the natural 
frequency of the diaphragm. Sustained vibra¬ 
tion was obtained by attaching a weight at the 
center and by drilling holes in the back electrode 
to reduce the air damping. It should be clearly 


understood that this is a radically different 
scheme from the first mentioned one of using 
a microphone whose deflection duplicates the 
pressure changes in the shock wave and whose 
motion ceases immediately thereafter. These 
two different systems are called respectively 
the resonant FEI and the aperiodic FEI. 

A very considerable amount of experimental 
work and field testing revealed that when res¬ 
onant diaphragms, having a Q between 50 and 
100 and natural frequencies of 1,600 and 2,400 c, 
respectively, were used, very peculiar response 
patterns were obtained. The response was not 
always a monotonically decreasing function with 
increasing miss distance and the peculiarities 
were dependent also on the caliber. Both in¬ 
dividual and sum response patterns were very 
irregular in shape. In one design having a 
cylindrical case in which the diaphragms were 
not flush with the ports at the ends but com¬ 
municated with the external air through a 
canal about 1 in. in diameter and ^ in. deep, 
the response could be almost completely sup¬ 
pressed by changes in the geometry of this canal. 
The canal was formed by inserting a sponge- 
rubber washer to fill the clearance between the 
case and the face of the microphone frame so 
that the sound entered through the hole in the 
washer. With this sponge rubber in place, nor¬ 
mal response was obtained but upon its removal, 
so as to leave a large internal annular space 
around the entry port, response was almost 
completely suppressed. These effects and others 
led to the suspicion that the ballistic shock wave 
must have more sharply defined periods asso¬ 
ciated with its wave form than at first supposed. 
After fundamental research had revealed the 
N-shaped shock-wave pressure profile and its 
variation in period with miss distance, theoreti¬ 
cal calculations as to the effect of such a tran¬ 
sient on vibrating systems such as the high-Q 
diaphragms, led to the prediction of null re¬ 
sponse points at definite miss distances for each 
caliber and diaphragm frequency. These null 
response nodes were then experimentally veri¬ 
fied by actual shooting. The theoretical solutions 
for the response of high-Q diaphragms to N- 
wave transients were also checked in the lab¬ 
oratory by means of the electrostatic micro¬ 
phone tester used in conjunction with an elec- 



SUMMARY OF DEVELOPMENT 


59 


tronic circuit especially designed to generate 
electric N-wave transients of adjustable period 
which could be applied to the high-Q condenser 
microphones as an electrostatic driving force. 
The destructive interference at the critical null- 
response periods of the N wave could be clearly 
observed on the oscilloscope screen. It became 
evident that the curve of response of the res¬ 
onant diaphragms as a function of miss dis¬ 
tance exhibited nodes and loops so that to a 
given response level there might correspond 
three or even more different miss distances. 
Such an ambiguity is obviously very undesirable 
in scoring gunners, especially when it is recalled 
that in towed flight the doppler change in 
shock-wave period would greatly increase the 
complexity of this situation. 

For these reasons, early in 1944 intensive 
work was started on the development of the 
aperiodic type of FEI, which has become the 
final design. For the reasons we have outlined 
above, this requires two distinct r-f channels 
between transmitter and receiver, one for each 
microphone. 

The “Noise” Problem and Its Treatment 
in the Aperiodic FEI System 

At the same time it was decided to take ad¬ 
vantage of the very high audio frequencies pres¬ 
ent in the ballistic shock wave as a result of the 
sharp discontinuities in the wave form, to ob¬ 
tain improved freedom from so-called “noise” 
disturbances. It was not certain at the time that 
insertion of a filter in the receiver to cut off 
low frequencies would improve the signal-to- 
noise ratio. It was merely felt that the two ex¬ 
tremely sharp discontinuities in the N-wave 
profile constituted something so radically dif¬ 
ferent from ordinary sounds that possibility of 
such an improvement seemed a good gamble. 
The peak amplitudes in the N-wave discontinu¬ 
ities (i.e., the pressure steps) appeared to be 
the most desirable characteristic of the profile 
to use as a measure of the miss distance, and 
oscillograms taken in static firing indicated sur¬ 
prisingly good shot-to-shot reproducibility of 
these pressure-step amplitudes. One reason why 
noise elimination by restriction to higher audio 
frequencies seemed promising was that a com¬ 
parison between even the 1,600- and 2,400-c 


resonant microphones frequently seemed to 
show a favorable reduction in signal-to-noise 
ratio for the higher frequency. It should be 
explained that the sources of noise in the FEI 
are far from being entirely acoustic sounds 
reaching the microphone by air waves. There 
seemed strong reason to believe that mechanical 
vibration of the resonant microphones (with 
their weight-loaded diaphragms) from the flap¬ 
ping of the towed target, exerted inertia forces 
on these diaphragms. For this reason, extremely 
soft shock mounting of the microphones and 
their supporting structures had to be provided 
in the resonant system. In the aperiodic system 
the insertion of an audio filter cutting off fre¬ 
quencies below 4,000 c has produced a very great 
improvement in the signal-to-noise ratio and 
also has permitted complete elimination of the 
shock mounting, a fact which indicates that a 
large contribution to the noise difficulty did in¬ 
deed come from the mechanical vibration of 
the microphones. Other possible sources of 
noise interference in the low-frequency range 
may have been flapping of the radio antenna 
with consequent fluctuations in capacity which 
reacted upon the r-f circuits to give frequency 
modulations in the low audio frequencies. To 
avoid this a shielded master oscillator power 
amplifier [MOPA] transmitter was developed in 
which changes in antenna capacity and loading 
could react on the frequency-determining r-f 
oscillator to a minimum degree. (Crystal fre¬ 
quency control was not readily feasible since, 
to keep the transmitter simple, the condenser 
microphone capacities were shunted across the 
oscillator tank circuit to give the frequency 
modulations.) 

Sum-Response Zones and 
Difference-Response Lobes 

It was found desirable at an early stage to 
utilize the separate responses of the two micro¬ 
phones at the receiving station in the following 
way. Pulse-lengthened signals proportional to 
these responses were electrically added to form 
what is known as the sum response which is 
taken as a measure of the miss distance. The 
difference between the two microphone signals 
or some other indication as to which of the two 
was the greater, was taken as an indication of 



60 


THE ACOUSTIC FIRING ERROR INDICATOR 


the side of the transmitter on which the bullet 
passed. In the final Model XI-A the receiver cir¬ 
cuits provide two directionality-indicating chan¬ 
nels corresponding to the two microphones in 
the transmitter. The coupling is such that a 
signal appears at the output only in that chan¬ 
nel which receives the earlier of the two signals, 
that from the other channel being completely 
blocked. The signal which does appear is a 
measure of the response of that microphone 
alone. 

Adjustable thresholds are provided in the re¬ 
ceiver in such a way that the sum signal is, 
according to its intensity, routed into one or 
more of three different sum channels. A thresh¬ 
old is also set below which the directionality 
signal produces no effect (to avoid spurious sig¬ 
nals from accidental low-level disturbances). 
Choice of these thresholds therefore delineates 
in the target plane certain areas around the 
transmitter which in the aperiodic type are, for 
the sum response, substantially concentric cir¬ 



cular zones. Ideally the passage of a bullet in 
such a zone generates a signal capable of sur¬ 
mounting a given threshold in the receiver, and 
therefore excites a recording mechanism to reg¬ 
ister the event. The directional-response thresh¬ 


old defines an area in the target plane on one 
side of the transmitter known as the directional- 
response lobe or difference-response lobe. Pas¬ 
sage of shots within the boundary of this area 
will be recorded as a miss on that side of the 



DIRECTIONAL RESPONSE LOBES (40mm) 

ASPECT ANGLE 10°(TRAJECTORY 10° OFF 

NORMAL TO MICROPHONE AXIS) 

BULLET SPEED 2260FT/SEC 

Figure 14. Directional-response lobes. 

transmitter. It is desirable to minimize as 
much as possible the variation in the dimensions 
of these lobes as the aspect angle of the trans¬ 
mitter changes relative to the direction of shoot¬ 
ing. 

Figures 13 and 14 show an example of the 
zones and lobes for 40-mm fire. The unbalance 
in the areas of the lobes for a 10-degree aspect 
angle can be seen. In Appendix II 2 a complete 
analysis of such distortions of pattern is given. 

Defects of the Aperiodic FEI 

Listed are the sources and types of error or 
defect which are to be minimized or avoided 
in the aperiodic FEI, here summarized before 
a description of the design itself is given. 

1. Noise. By tfiis meant all forms of spu¬ 
rious excitation of the FEI system from sources 
other than ballistic shock waves such as wind 
noise, flag or sleeve flapping, excitation of the 
microphone diaphragms by mechanical vibra¬ 
tion, transmitter-antenna flap producing spu¬ 
rious frequency modulation, interference from 
other radar wave, static, and gasoline-engine 
ignition. 

2. Instability of transmitter carrier fre¬ 
quency. 
























SUMMARY OF DEVELOPMENT 


61 


3. Doppler effect errors. These will be pres¬ 
ent only in a system whose response is depend¬ 
ent on the period T f of the shock wave. 

4. Delay errors. By this is meant the error 
which comes from the time delay between the 
instant of the piercing of the imaginary target 
plane by the bullet and the later instant of ar¬ 
rival of the shock wave at the microphone. Since 
the target containing the microphone is in mo¬ 
tion through the air the miss distance indicated 
by the system will apply to the later of these two 
instants though it is the earlier of the two for 
which the information is desired. The dominant 
factor controlling the magnitude of this error 
is the ratio of the velocity of the target to the 
velocity of sound. (See Appendix II. 2 ) 

5. Aspect angle errors coming from the vary¬ 
ing aspect angles at which the microphones are 
presented to the shock waves for different di¬ 
rections of towing and shooting. 

6. Unit-to-unit reproducibility of microphones 
and transmitters as to acoustic sensitivity. 

7. Round-to-round reproducibility of the 
shock waves themselves. 

8. Errors of receiver response. 

9. Insufficient radio-signal strength limiting 
operating distance from transmitter to receiver. 

Measures to Remedy Aperiodic 
FEI Defects 

The following measures were taken to sur¬ 
mount, eliminate, or avoid each of the difficulties 
mentioned in Section 2.6.2. 

1. Noise. The design of the aperiodic micro¬ 
phone (to be described presently) coupled with 
the use of an audio filter in the receiving station, 
has made it possible greatly to attenuate audio 
frequencies below 4,000 c while permitting the 
peak amplitudes of the shock waves to be faith¬ 
fully transmitted with much less attenuation. 
The insertion of a band-pass filter in the circuit 
improves the signal-to-noise ratio by a factor 
9 at 225 mph towing speed and by a factor 12 
at 150 mph. By using the filter with speeds up 
to 225 mph, the peak-signal level for .50 caliber 
at 20 yd miss distance is more than 10 db above 
the noise, while for 40 mm this statement applies 
out to 40 yd miss distance. 20 ’” This has effec¬ 
tively eliminated all forms of acoustic and vibra¬ 
tional noise interference even as compared to 


shock-wave signals for the largest target zones 
requested. Frequency-shift noise from antenna 
flap has been corrected by the use of a shielded 
MOPA type of transmitter. Noise from out¬ 
side interference has been suppressed by im¬ 
provements in signal strength and in the an¬ 
tenna design of both receiver and transmitter. 
Interference from ignition systems especially 
in air-to-air applications is still a serious prob¬ 
lem for individual study on each ship, for which 
relatively obvious solutions have been found. 

2. Instability of transmitter frequency has 
been effectively solved by the shielded MOPA 
transmitter design. 

3. Doppler effect errors are suppressed by the 
design of the aperiodic system with its special 
audio filter which makes the response independ¬ 
ent of shock-wave period, T', to the required 
degree of accuracy for periods of 0.3 to 1.5 milli¬ 
seconds. 

4. The delay error can be eliminated to the 
first order, for the case of an aperiodic trans¬ 
mitter when and only when the microphone axis 
is parallel to the direction of tow. This is ac¬ 
complished by an appropriate unbalancing of 
the responses of the two channels through gain 
adjustments in the receiver, the amount of such 
unbalance being dependent on towing speed. 
The towing speed, however, must not be too 
large a fraction of sonic velocity (250 mph) to 
prevent second-order delay errors from becom¬ 
ing important. This is probably the most seri¬ 
ous limitation. The delay error if uncorrected 
makes the FEI transmitter less sensitive to 
shots placed behind the moving target than it 
is to those placed ahead. The zones of sum re¬ 
sponse without delay-error correction form a 
target which is somewhat decentered in the 
forward direction. An analysis of delay error 
and its correction is given in Appendix II. 2 

5. Aspect angle errors as regards sum re¬ 
sponse have been almost completely eliminated 
by the use of two aperiodic microphones mount¬ 
ed at opposite points of a rigid spherical en¬ 
casement with their diaphragms nearly flush 
with the outer surface. The diffraction of the 
shock waves around this obstacle can be shown 
from theoretical acoustics to have such prop¬ 
erties that the sum of the responses of the two 
microphones is almost completely independent 




62 


THE ACOUSTIC FIRING ERROR INDICATOR 


of their orientation. Thus for the purpose of 
scoring radial miss distance, the aspect angle 
error is practically negligible. This does not 
apply to directional indication however. 

The apex angle of the shock-wave cone places 
definite limits on the aspect angles over which 
directional indication is reliable. Beyond this 
range, directionality will be indicated in reverse 
sense. The upper three figures in Figure 15 
illustrate the geometry which controls these 
limits. Call this the reliable range of aspect 

LIMIT OF CORRECT 

DIRECTIONALITY DIRECTIONAL DIRECTIONALITY 

CORRECTLY INDICATED INDICATION REVERSED 



CORRECT DIRECTIONAL INDICATIONS 

DIR ECTION OF TO W 

ILLUSTRATING HOW THE FORWARD DIRECTIONALITY 
LOBE IS EXAGGERATED AT THE BEGINNING OF A 
TRANSVERSE COURSE AND THE REARWARD LOBE IS 
EXAGGERATED AT THE END OF THE COURSE 

Figure 15. Limits of correct directionality and changes 
in lobe size with aspect angle. 

angles, i.e., bullet trajectories falling roughly 
not more than 30 degrees either side of the 
perpendicular to the microphone axis. 

The dimensions of the lobes of directionality 
response are still unavoidably dependent upon 
aspect angle, even inside the reliable range. 
This does not mean that inside the reliable range 
any single report as to firing-error directional¬ 
ity given by the FEI is incorrect. It merely 
means that for the purpose of indicating wheth¬ 
er a shot fell fore or aft of the moving target 
the zone of sensitivity within which any indica¬ 
tions are given at all will be greater around the 
microphone which is turned toward the gun. 
Although every indication given as to direction¬ 
ality may be correct, a gunner may appear to 
miss (for example) more often on the lagging 
side, if during his shooting the lagging lobe is 
larger than the leading lobe, simply because no 
report is given for a larger fraction of his 
leading misses than for his lagging misses. The 


lower views in Figure 15 illustrate how the 
leading lobe predominates in the beginning half 
of a transverse course while the lagging lobe 
predominates in the later half because of 
change of aspect angle. To summarize, it may 
be said that directional indications are not mis¬ 
leading for the informing function but are mis¬ 
leading if an attempt is made to score direction¬ 
ality of misses statistically. 

6. The problems of manufacturing produc¬ 
tion and adjustment of microphones and trans¬ 
mitters to give unit-to-unit reproducibility of 
acoustic sensitivity to within 6 or 7 per cent 
have been solved by a very long and painstaking 
experimental production study of all the steps 
in manufacture and by many new techniques. 
Complete descriptions of all these procedures, 
techniques, and test methods are given in Ap¬ 
pendix IV. 4 This was one of the most difficult 
problems in the entire project. 

7. A great deal of static-firing data has proved 
the shot-to-shot reproducibility of shock-wave 
transmission to be better than expected (of the 
order of 6 or 7 per cent) and adequate for the 
present purpose. This is a fortunate peculiarity 
which comes presumably from the very large 
amplitudes of ballistic shock waves relative to 
ordinary thermal disturbances in outdoor air. 
The reproducibility is in all probability much 
poorer in towed flight because of air turbulence 
and other disturbances set up by the towed 
target. The results of validation tests in towed 
flight indicate that zone boundaries are much 
less sharply defined than in static firing. Never¬ 
theless, much of this averages out statistically 
as the validation tests show (see below). 

8. Receiver-response errors have been re¬ 
duced to negligible proportions, provided the 
zone adjustments are checked at least once a 
month with the “E” checking equipment. In 
the case of airplane receivers subject to vibra¬ 
tion more frequent checking is recommended. 

9. Much attention has been given to antenna 
design and matching problems, both in case of 
receiver and transmitter, to give optimum r-f 
transmission distance. For ground-to-air fire, 
where greater distances are needed than in the 
air-to-air case, special receiving antennas have 
been designed with parasitic elements which 
increase the reception in the desired direction. 










SUMMARY OF DEVELOPMENT 


63 


2 ' a ' 4 The Aperiodic FEI Briefly Described 

A more complete description of the aperiodic 
FEI with technical information will be given 
in Section 2.6. The brief description here is 
merely for a clear understanding of the com¬ 
ponents of the device and the functions they 
perform. There are two main components, a 
transmitter located in the airborne target and 
a receiver, usually near the gun. Transmitter 
and receiver are linked by two radio carrier 
frequencies (e.g., 55.5 and 56.75 me). 

The Aperiodic FEI Transmitter 

The transmitter, located in the airborne tar¬ 
get, has already been pictured in Figures 1 and 
2. Its spherical case is provided with two op¬ 
posite openings from which the aperiodic micro¬ 


phones face with their diaphragms nearly flush 
with the surface. Protecting screens cover the 
openings. The diaphragms deflect in response 
to the shock waves from the bullets, the re¬ 
sponse depending on the obliquity with which 
the waves impinge. The sum of the peak deflec¬ 
tions is, however, substantially independent of 
this obliquity and depends only on the distance 
from the bullet trajectory. Each of the con¬ 
denser microphones controls the frequency of 
a radio-oscillator circuit so that the shift from 
the undisturbed frequency is a measure of the 
diaphragm deflection. Each microphone thus 
frequency-modulates a separate channel of com¬ 
munication between the transmitter and its 
receiving station. The two transmitters are op¬ 
erated by batteries running 2 to 4 hours on one 
charge. 



Figure 16. Model XI-A receiving station. 












64 


THE ACOUSTIC FIRING ERROR INDICATOR 


The FEI Receiving Station 


A picture of the receiving station, Model 
XI-A, is shown in Figure 16. This station con¬ 
sists of two distinct f-m receivers each of which 
must be tuned to the corresponding microphone 
channel in the transmitter. Once tuned, how¬ 
ever, automatic frequency control [AFC] is 
provided to maintain it. The two f-m receivers 
(on the right-hand drawers in Figure 16) con¬ 
vert the shock-wave signals into electric voltage 
impulses which are measures of the peak ampli¬ 
tudes in the discontinuities of the N-shaped 
profiles. The audio section of the receiver (the 
left in Figure 16) contains the scoring circuits 
which sum the signals from the two micro¬ 
phones, classify this sum signal into the ap¬ 
propriate zoning channels, according to its in¬ 
tensity, and generate appropriate output pulses 
to drive counters or a tape recorder indicating 
the position of the shot in the radial zone of the 
target. The audio section also contains two 
channels to indicate on which side of the trans¬ 
mitter the shot was placed according to which 
microphone was excited first. This is indicated 
by impulses which may actuate either dial 
counters or a recording tape. 

The receiver requires 110-v 60-c power to run 
it. For aircraft applications, therefore, where 
the primary power supply is 24-v direct current, 
appropriate inverters must be available. For 
this purpose a 0.75-kva 115-v 400-c inverter 
running on 24-v direct current has been used 
with very satisfactory results. 

Tape Recorder 

The electrographic tape recorder is shown in 
Figure 17. A motor-driven roller propels 1 in. 
wide, damp electrographic recording paper at a 
rate of approximately 1 in. per second. The 
paper feeds off a 200-ft spool, and the records 
are made by electrolytic action of 7 pens riding 
on the paper which is ejected through the front. 

The pens are of platinum supported by steel 
leaf springs. The recording paper must remain 
moist in order for the electrolytic action to take 
place. The platinum tips make no mark until a 
current pulse passes through the pen and paper 
into the metal roller. The dots on the paper show 
instantaneously which zones were excited, thus 
indicating both the miss distance and direction- 



Figure 17. Electrographic tape recorder. 


ality of each round which comes within the re¬ 
sponse pattern. 

Two of the pens are brought to outside ter¬ 
minals as spares. These are often useful to 
record on the tape the instant when either of 
two guns is fired. The signal comes from micro¬ 
phones placed near the muzzles or by set-back 
switches operating on the gun recoil. This 
makes identification of the marks on the tape 


b ui.iiTifiu'i'i'ffr' 1 







SUMMARY OF DEVELOPMENT 


65 


much easier and gives an independent record 
each time a shot is fired even though the miss is 
too great to actuate the FEI. By comparison of 
such records with the FEI record the time of 
flight of the bullet can be obtained and the range 
is thus easy to ascertain. 

The advantage of the tape recorder is that it 
gives a complete recorded history, round-by- 
round, of the errors of fire. The counters merely 
give, at any instant, totals of the number of 
rounds in each zone or directionality lobe since 
the counter was reset. Unless records are being 
continually jotted down from the counters by 
a very agile observer, information is lost as to 
the order in which the misses occurred or as 
to whether there was a tendency to lead at the 
start and lag at the end, etc. For scoring pur¬ 
poses, however, the counters alone are probably 
adequate although a dial counter has not as yet 
been developed whose mechanical action is as 
positive and as free from trouble and the need 
of servicing as is the electrolytic action of the 
recorder. 

Counter Chassis 

A counter assembly can be connected to the 
recorder output of the receiver. This assembly 
contains five dial impulse counters each of which 



Figure IS. Dial counter assembly. 



Figure 19. Photograph of complete receiving station 

at Ft. Bliss. 

totalize the impulses in its channel, giving rec¬ 
ords of three miss-distance zones and two di¬ 
rectionality lobes. Each counter has two indi¬ 
cator hands which can thus totalize 6,000 counts 
without resetting. The dials counting the num¬ 
ber of rounds falling in the three radial miss- 
distance zones give a measure of the gunner’s 
marksmanship. 

The dial readings counting the number of 
rounds falling in the directionality (lead or lag) 
lobes have to be interpreted with care since, as 
explained before (see Section 2.5.3) the sizes of 
these lobes vary with the aspect angle of the 
transmitter relative to the trajectory, so that 
while no single directionality indication may 
be incorrect the trend of a gunner as to direc¬ 
tion of miss may be misleading. 

Figure 18 shows the counter assembly. The 
present FEI receiving station will not drive 
counter and tape recorder simultaneously, but 
either can be plugged into the receiver. 

For details regarding the dial counters see 
Section 2.6.6. 

General Appearance of 
the Receiving Station 

Figure 19 shows an FEI receiving station set 
up for work in the field at Ft. Bliss, Texas. In 
this picture there appear the power supply, the 
receiver proper (Model XI instead of XI-A), 
the tape recorder, a small special chassis pro¬ 
viding informing lights without counters, and 










66 


THE ACOUSTIC FIRING ERROR INDICATOR 


a special amplifier chassis. The latter amplifies 
the output of carbon microphones set up near the 
gun muzzles to furnish on the tape recorder the 
occurrence of each muzzle blast. The antenna, 
in this case of the directional (parasitic) type, 
also appears in this photograph. 

Validation Tests of the FEI 
in Towed Flight by Means of Cameras 

The aperiodic FEI has passed ground-to-air 
validation tests for its accuracy in reporting 
radial miss distance when used in towed flight 
mounted in a flag target. The tests were con¬ 
ducted at Ft. Bliss, Texas, during May and June 
of 1945. The placement of the shots relative to 
the target was observed with the Stibitz dual 
photographic theodolite [SPT] and an FEC de¬ 
veloped especially for the purpose. This latter 
has been described in Section 2.4.2 and pictured 
in Figure 12. On the FEC film the instant of 
arrival of the shock-wave signal at the trans- 

DIRECTION Or TOW-•- 



LEGEND INDICATED BY FEI 
-- AS ZONE I (0 - 4 YD ) 

• AS ZONE H(4-I5YD ) 

° AS ZONE EE (I5-25YD ) 

Figure 20. Combined plot of firing of May 21 and flak 
analysis firing of May 28—June 1. FEI versus FEC. 
Location: Ft. Bliss, Dona Ana Range; caliber: 40 mm; 
number of rounds plotted: 269. 

mitter is indexed to determine when the tracer 
bullet pierces the target plane. Plots were made 
in which the actual position of each shot in the 
target plane as ascertained by the camera was 
plotted to scale. A report on the same shots by 


the aperiodic FEI as to the nominal miss-dis¬ 
tance zone in which each shot fell (i.e., 0 to 4 yd, 
4 to 15 yd, or 15 to 25 yd for 40-mm fire) was 
indicated by a circle, dot, or triangle at the point 
in question. The circular nominal zone boundar¬ 
ies were drawn to scale on these plots for com¬ 
parison with the FEI report. Three plots of 
this type are shown in Figures 20, 21, and 22. 
It will be noted that a certain number of shots 
actually falling outside a given zone boundary 

DIRECTION OF TOW- - 



LEGEND INDICATED BY FEI 
*-AS ZONE I(0-2.5YD ) 

• -AS ZONE 11(2.5-5 YD ) 

° -AS ZONE HI (5- 10 YD ) 

Figure 21. Correlation of FEI with FEC. Location: 

Ft. Bliss, Hueco Range, June 4-5, 1945; caliber: .50; 

number of rounds plotted: 120. 

are recorded as inside and vice versa. This 
lack of sharp definition of the zone boundaries 
is much more in evidence for the case of towed 
flight than for the case of static firing, for 
reasons discussed in Appendix II 2 under the 
heading of “Errors from Noise.” Figure 23 is 
a shot diagram taken in static fire to illustrate 
this point by comparison with Figures 20, 21, 
and 22. (For a more complete account of such 
static shot-response patterns see OSRD 4664. 28 ) 
Obviously less weight should be attached to a 
wrong report by the FEI as to radial zone if 
the shot is just outside the zone boundary but 
close thereto than if it is farther away. Further¬ 
more, errors in reporting miss distance are less 
serious for large miss distances than for closer 
hits. Therefore some quantitative procedure 
for scoring the FEI against the camera must be 
worked out if a single figure is to be assigned as 
a measure of the accuracy of the FEI. This 
procedure is described now. A more complete 
account and analysis of the camera-validation 
tests has been given in OSRD 5553. 32 










SUMMARY OF DEVELOPMENT 


67 


direction of tow-*- 


25YD 



LEGEND INDICATED BY FEI 
■»-AS ZONE 1(0-4 YD ) 

• - AS ZONE K4-I5YD ) 

° - AS ZONE 3T(I5-25YD ) 

Figure 22. Combined plot of filing of May 21 and flak 
analysis firing of May 28-June 1. FEI versus Stibitz. 
Location: Ft. Bliss, Dona Ana Range; caliber: 40 mm; 
number of rounds plotted: 295. 

Method of Computing Scores and FEI 
“Accuracy” by Harmonic 
Mean Miss Distance 

The numbers of rounds reported by the SPT 
in each concentric radial zone (as, for example, 
Zone 2 between radii 4 and 15 yd) are tabulated 


in Table 4 for the three zones and outside Zone 
3. In the SPT column the 170 shots in this latter 
category were simply those observed to fall out¬ 
side the 25-yd radius. All of the 537 rounds dis¬ 
cussed were therefore located by the SPT 
camera. The 193 rounds in the FEI column is 
the difference between the total rounds and the 
sum reported by the FEI in its three zones. 
There is also tabulated under the label Target 
Scores the number of rounds falling inside the 
three circular areas of radii 4 yd, 15 yd, and 25 
yd, respectively, by the FEI and by the SPT for 
comparison. The tabulations are expressed both 
as hits and as per cents of the 537 total rounds 
photographed by the SPT. It will be noted that 
there is satisfactory agreement between the per 
cent scores as given by the FEI and by the 
validating SPT camera. 

In order to obtain a single figure of merit with 
which to express the accuracy of the FEI in re¬ 
porting miss distance, the harmonic mean miss 
distances from the FEI zone reports and from 
the SPT zone counts for comparison were com¬ 
puted. To do this the number of shots reported 
in each zone by either device is divided by the 
mean radius of that zone, the three quotients 
are added and the sum is divided by the total 
number of shots. The reciprocal of this result 
is the harmonic mean miss distance. 



Figure 23. Shot pattern in static fire, 40 mm, Camp Irwin. 































68 


THE ACOUSTIC FIRING ERROR INDICATOR 


Table 4. Scoring data, Ft. Bliss, 40 mm, May 21 and May 28 to June 1, 1945 



Zone Scores 

FEI 

No. of hits 

Score % 

SPT 

No. of hits 

Score % 

Zone 1 : 0-4 yd 

45 

8.4 

45 

8.4 

2 : 4-15 

169 

31.5 

205 

38.2 

3 : 15-26 

130 

24.2 

117 

21.7 

Outside 3 : 25- 

193 

35.9 

170 

31.7 

Total 

537 


537 


Harmonic mean miss: FEI 11.49 yd; SPT 10.77 yd. 




Per cent disagreement: 7% 






Target Scores 





FEI 


SPT 



No. of hits 

Score % 

No. of hits 

Score % 

0-4 yd 

45 

8.4 

45 

8.4 

0-15 

214 

39.9 

250 

46.6 

0-25 

344 

64.1 

367 

68.3 


Number of transmitters used 

On the 40-mm scoring, 7 transmitters were used on five different days. 
On the .50-cal. scoring, 5 transmitters were used on two different days. 


Comparison of Zone Scores of FEI and SPT 

It will be noted from Table 4 that the FEI 
reported a harmonic mean miss distance of 
11.49 yd, while the SPT reported 10.77 yd. The 
difference, an error of 7 per cent may be taken 
as an expression of the accuracy of the FEI if 
no error whatever exists in the SPT. This meth¬ 
od amounts to weighting the number of shots 
in a given zone in inverse ratio to the mean 
radial miss distance of the zone. This procedure 
is rational since, down to a certain lower limit, 
accuracy in reporting closer shots is more valu¬ 
able and should receive greater weight. This 
statement is true, of course, only down to a 
certain lower limit of miss distance. The inner¬ 
most zone (e.g., 2.5 yd for caliber .50) may be 
roughly identified with this lower limit. There 
is probably less value in discriminating between 
shots as to miss distances of a few feet since 
such differences can come from many compli¬ 
cated ballistic causes not connected with marks¬ 
manship. It is also reasonable as being quite 
closely related to what the FEI actually meas¬ 
ures, namely the amplitudes of the shock waves 
which are nearly in inverse proportion to miss 
distance. 

Comparison of Target Scores 
of FEI and SPT 

If examination is made of target scores it 
is seen that the largest circular target area of 
25-yd radius was of such size as to permit a 


score of over 60 per cent hits. For rating gunners 
most reliably and economically from the point 
of view of minimizing statistical fluctuations of 
score this size is the one to be preferred over the 
others. 30a For this size, the absolute difference 
in score between the FEI and SPT was 4 per 
cent, an amount which is about 6 per cent of 
the score itself. 

Probability Curves for FEI Zones 

As an alternate method of indicating the per¬ 
formance of the FEI, there is shown in Figure 
24 a plot, from the camera validation data, of 



Figure 24. Zone-recording probability. The ordinate 
of each curve gives the probability that a shot at the 
given miss distance will be recorded as being in that 
zone. Location: Ft. Bliss, Dona Ana Range, May 21, 
May 28-June 1, 1945; caliber: 40 mm. 


the probability, as a function of radial miss dis¬ 
tance, that the FEI will report a 40-mm shot as 
falling in each one of the three zones. A fourth 
curve indicates the probability of failures to 






























































SUMMARY OF DEVELOPMENT 


69 


report. The nominal boundaries of the zones are 
marked with heavy vertical lines. 

It should be noted from this plot that the FEI 
may report a shot as in a given zone which was 
shown by the photographic theodolites to have 
fallen elsewhere. Also the FEI may fail to re¬ 
port a shot which was actually placed inside a 
given zone boundary according to the theodo¬ 
lites. These two effects have a mutually can¬ 
celling tendency, the net result being an un¬ 
sharpness of the zone boundaries. It is this 
increased unsharpness of the zone boundaries 
in the case of towed flight as compared to static 
firing which leads to the belief that the shot-to- 
shot reproducibility of the shock waves as re¬ 
ceived at the microphones is considerably im¬ 
paired by the turbulence of the air and other 
disturbances associated with towing. The re¬ 
sults in Table 4 show that for statistical pur¬ 
poses of scoring, however, these effects tend to 
average out in a very reasonably small number 
of rounds, so as to leave no serious error in the 
score. For the number of rounds fired the uncer¬ 
tainty in the score so introduced by the FEI is 
far less than the statistical uncertainty in rat¬ 
ing a gunner by the number of holes he could 
have made in a flag on the same number of total 
fired rounds. Unfortunately no record of the 
number of direct flag hits is available but it is 
easy to compute, from the mean density of shots 
recorded by the SPT inside the 4-yd radius, 
approximately how many holes could be ex¬ 
pected. This is only about 8 holes, a number so 
small that there is, according to statistical 
theory, about 30 per cent chance of its being in 
error either way by more than y/8 holes, or very 
nearly ±3 holes (i.e., the standard deviation). 
Thus the flag score for an expenditure of the 
same 537 rounds would stand a 30 per cent 
chance of being in error by ±35 per cent or 
more (y/8/8) of the score itself, as compared 
to the error of 6 per cent of the score with the 
FEI. 

As a final result, it may be stated from the 
Ft. Bliss tests that the FEI reported radial miss 
distance (i.e., harmonic mean value) with an 
error not greater than 7 per cent. This figure 
is based on the assumption of complete accuracy 
of the validating camera. This assumption is 
not completely true but although correction of 


camera errors, if it were possible, might reduce 
the figure of 7 per cent error in the FEI, it 
would probably not be a large effect because 
it is the squares of the standard deviations of 
the two methods which are additive. 

The accuracy of the FEI indicated by these 
tests is regarded as satisfactory by the using 
Services. The enormous gain in the number of 
shots whose proximity to the target can be 
recorded (in comparison to the number of direct 
hits on flags or other material targets) is the 
element of greatest advantage. In the case of 
direct hits the number of these is so small that 
the statistical uncertainty as to their signifi¬ 
cance is enormously larger than the 7 per cent 
error of the FEI. 

The FEI was used during the latter part of 
June 1945 by the 37th AA Brigade, Los Angeles, 
to determine the relative merits of different gun 
sights. As an index of its statistical superiority 
over the method of direct hits, it may be stated 
that in these tests, out of 103,768 rounds which 
were fired, 120 direct hits on the towed flags 
were recorded while the FEI furnished zone 
data on the proximity of 16,111 rounds. 

For a more complete report on the camera 
validation tests of the FEI in towed flight, see 
OSRD Report 5723. 34 

Limitations of the FEI 
as Developed to Date 
Limitations as to Application 

The duration of the work on this project was 
not sufficient to permit the development of the 
FEI for all applications which have been sug¬ 
gested or requested by the Armed Forces since 
its inception. As developed to date, the FEI has 
only been studied for use in towed flag targets 
and in gliders for ground-to-air firing and for 
air-to-air (bomber-to-fighter plane) firing. In 
the first case the calibers studied have been cali¬ 
ber .50, 20 mm, and 40 mm. In the second case 
only caliber .50 has been studied. Work was 
started on the application to small radio-con¬ 
trolled plane models as targets (the OQ model) 
and it was found by test on the OQ3 that no 
serious interference was to be expected from 
the sound of the motor. Time has not permitted 
further work on this very promising application. 

Requests and suggestions have been made for 




70 


THE ACOUSTIC FIRING ERROR INDICATOR 


development of the FEI for use with larger 
calibers such as 90 mm and at higher altitudes. 
It should be clearly understood that in the case 
of the larger caliber explosive projectiles the 
FEI in its present form would be suitable for 
indicating the shock wave from the passage of 
the unexploded shell past the aerial target but 
not suitable for indicating the burst. An FEI 
working on the burst would probably require a 
considerable amount of new fundamental re¬ 
search as well as development. Even for indicat¬ 
ing the passage of 90-mm shells by shock wave 
with the present FEI a considerable program 
of static firing would be needed. 

The question of calibration for high-altitude 
work would also call for a much extended pro¬ 
gram. To date only altitudes from sea level to 
about 8,000 ft (3,000 ft above Ft. Bliss, Texas) 
have been studied. Over this range there was not 
detected any significant change in calibration 
with altitude. Such constancy, however, cannot 
be guaranteed, without tests, at altitudes such 
as 20,000 ft. 

None of the applications of the FEI to train¬ 
ing of fighter pilots in fixed gunnery has been 
studied. 

A naval application of the FEI to the scoring 
of air-to-air rocket fire was initiated in 1944. 
Since the rocket differs widely in many respects 
(geometry, speed, ballistic constant, etc.) from 
the projectiles already studied and since also 
it was to be fired forward from a plane already 
moving at high velocity, this study, as pointed 
out in Section 2.3.5, called for the construction 
of a special 350-ft launching rail installation 
with a static-firing range to permit firing the 
rockets from initially moving rocket-propelled 
carriages under correctly simulated conditions 
for calibrating FEI transmitters. This range has 
been in process of construction by the Navy at 
the Inyokern Naval Ordnance Testing Station, 
but the pressure of more urgent work has de¬ 
layed it so that it was not ready for use in con¬ 
nection with this work. 

Limitations Imposed by Physical Conditions 

Besides the above-mentioned limitations, the 
FEI as at present developed has other limita¬ 
tions : 

1. Weather conditions such as rain, or ice, may 
prevail so that a coating of water or ice forms 


on the microphone diaphragms. Such coatings 
modify the calibration so as to cause erroneous 
results, as laboratory tests have shown. 

2. As stated above, calibration is unknown for 
altitudes above sea level greater than 8,000 ft. 

3. The spherical transmitter must be moun¬ 
ted on its target or other supporting vehicle in 
such a way that the shock waves from all bullet 
trajectories to be scored, wherever they may 
pass, can come directly to the transmitter sphere 
without encountering intermediate obstacles 
and can sweep on all sides of the sphere with as 
free clearance, all around, as possible. (Narrow 
rods, guy wires, or stays are less serious ob¬ 
stacles in this respect than large, flat rigid sur¬ 
faces.) Acoustic reflections from nearby sur¬ 
faces must also be carefully avoided. It has 
been frequently suggested, for example, that the 
two hemispherical parts of the transmitter be 
divided, placing the two halves as “blisters” on 
the opposite sides of the fuselage of a radio- 
controlled plane. Such a procedure would be 
fatal to the scoring accuracy of the present de¬ 
vice. A complete restudy by static fire to find 
the new shapes of the sum-response zones and 
their changes in shape with different aspect 
angles would be required. In all probability the 
results would be too complicated for practical 
use in the comparative scoring of marksmen. 

4. Radio reception distances in the present 
FEI can hardly exceed four miles under the 
best conditions yet encountered. This distance 
could, of course, be extended by a redesign of 
the transmitter with more powerful tubes, but 
this would result in much shorter battery life 
or considerably more battery weight to be sup¬ 
ported in the airborne target. In an application 
of the FEI to blast-pressure measurements 
from the atomic bomb (see Section 2.5.7) it was 
possible to support greater weight because the 
equipment was mounted in a parachute. Recep¬ 
tion distances of 10 miles were thus realized. 
An operating time, however, of only a few min¬ 
utes was required. 

5. The projectile speeds must be substan¬ 
tially in excess of the velocity of sound as they 
pass the target (preferably at least 1,400 ft per 
second). 

6. If the bullet trajectories make an angle 
with the microphone-pair axis differing too 



SUMMARY OF DEVELOPMENT 


71 


greatly from 90 degrees the directional indica¬ 
tion, as already explained, will be unreliable or 
even reversed. 

7. Towing speeds are probably limited (chief¬ 
ly by the delay error) to speeds not in excess of 
250 mph. The ratio of the towing speed to the 
velocity of sound is the chief controlling factor 
here. 

8. Attention is called to the discussion under 
Section 2.6.8. It is important to record here that 
any changes from the present physical design 
of microphone and transmitter may make it 
necessary to repeat the entire process of field 
calibrations by as extended a program of ex¬ 
perimental static firing (see Section 2.4.2 and 
Appendix V 5 ) as this project has already under¬ 
taken. In view of the amount of time, special 
field-measuring equipment, and experience that 
this has required, such a repetition is an impor¬ 
tant consideration not to be taken lightly. 
Changes, even though they promise minor im¬ 
provements, must be considered with this in 
view. 

Application of the FEI to the 
Measurement of Blast Pressure 
from the Atomic Bomb 

Work was begun in April 1945 to utilize the 
FEI as an airborne pressure gauge over enemy- 
held territory. The purpose of the measurement 
was to determine the efficiency of the atomic 
bomb in combat by comparing amplitudes of 
the pressure waves with those from known 
amounts of TNT. 

Apparatus 

Transmitter. A standard FEI transmitter is 
used to drive a power amplifier feeding approxi¬ 
mately 30 watts to a linear antenna. Only one 
half of the standard spherical unit was used. 
This considerable simplification of the FEI 
could be made in this special application because 
the conditions of the measurement were such 
that the angle of approach of the shock wave 
to the microphone hemisphere was known be¬ 
forehand for reasons soon to become clear. 

The hemispherical single microphone-trans¬ 
mitter unit is carried facing downward on the 
bottom of a cylindrical container about 36 in. 
long carrying the power stage and batteries. 
This assembly is dropped by chute from the 


bomber (with the microphone facing down¬ 
ward) at about the time the bomb itself is re¬ 
leased- While the unit is still in the bomb bay 
the transmitter is operated by the airplane gen¬ 
erators to save the battery; but as soon as it is 
dropped, power supply is switched to the in¬ 
ternal batteries. These need then operate the 
transmitter for only a few minutes. By this 
means as well as by the use of much larger bat¬ 
teries it was possible to increase the transmis¬ 
sion distance up to 10 miles. Before use, the bat¬ 
teries are kept warm by electrical heating as 
the altitudes for this work were considerable. 
The temperature and current drain are such 
that the output power is halved in iy 2 minutes 
after dropping. 

It turns out that the microphone sensitivity 
required here is the same as in regular FEI 
work. The microphone was therefore of stand¬ 
ard construction except for a calibrating device 
consisting of a piston, operated by a time fuse, 
which raises the pressure in the microphone 
chamber behind the diaphragm by a predictable 
amount. The resulting signal, timed to occur 
very shortly before arrival of the expected blast, 
thus offers an overall calibration. 

Receiver. The signals were received with the 
receivers originally manufactured for use with 
resonant transmitters. It was possible to use 
these resonant-system receivers because only 
one channel of information was needed. The 
special audio filter of the aperiodic receivers 
was omitted since the noise and mechanical vi¬ 
bration incident to towed flight were absent in 
this parachute application and it was desired 
to obtain an oscillographic record of the com¬ 
plete pressure wave profile, not just its discon¬ 
tinuities. The receivers were modified to feed 
a recording oscilloscope using a 3-in. blue screen 
tube situated in the bombing plane. Records 
were taken with a 16-mm, continuous motion 
camera. The entire system is direct-current 
coupled in such a way that even very slow im¬ 
pulses are recorded. 

Performance 

The airborne pressure gauges were used at 
an experimental explosion of 100 tons of TNT in 
June 1945. Excellent records were taken, in 
complete agreement with theoretical pressures. 

No records were taken during the explosion 



72 


THE ACOUSTIC FIRING ERROR INDICATOR 


of July 16, 1945, at the Alamogordo Air Base, 
N. M., because of poor weather conditions. 

Excellent pressure records were obtained 
over Hiroshima and Nagasaki. The estimates 
of “equivalent tons” of the bombs dropped on 
Japan are based on these records. 

26 DESCRIPTION AND TECHNICAL 
INFORMATION 

2-61 Experimental Production Program 
Two facts made a rather extensive program 
of experimental production unavoidable. These 
were (1) the expendable nature of the FEI 
transmitter units when used in flag targets and 
gliders, and (2) the fact that the problem of 
obtaining a high degree of unit-to-unit repro¬ 
ducible sensitivity in the transmitters was a 
paramount one to be solved. A design could not 
be “frozen” for quantity production until all 
the pertinent problems had been solved and 
until procedure had been standardized so that 
all the units manufactured would meet the re¬ 
quirements established and a sufficient number 
of satisfactory units had to be adequately 
tested by the contractor’s personnel and by the 
using Services. In all, some 460 aperiodic trans¬ 
mitters (each containing two microphones and 
two radio transmitters) were constructed, and, 
in addition, 295 single transmitter units of very 
special type were constructed for the Manhat¬ 
tan District for the study of atomic-bomb blast 
pressures. Beside these, a large number of ex¬ 
perimental types were constructed earlier, be¬ 
fore the satisfactory design was determined. 

Twelve distinct models of the FEI receiving 
station were designed, and several were con¬ 
structed to permit simultaneous experiments 
with them at widely separated military loca¬ 
tions designated by the different interested 
Service branches. In all, about 15 receivers of 
the various models were constructed in this ex¬ 
perimental production. 

The experimental production was carried on 
under Contract OEMsr-600 on the campus at 
CIT. 

The Aperiodic FEI Microphone 
This all-important component, whose tech¬ 
nical details of manufacture are more fully de¬ 


scribed in Appendix IV, 4 is shown in Figure 25. 
The exploded (top) view shows the five parts: 
the lock nut, the stretching button consisting 
of a threaded brass ring with insulated back 



Figure 25. Two views of aperiodic FEI microphone 
(exploded and assembled). 

electrode mounted on ceramic (steatite) insert, 
the frame, the beryllium copper diaphragm 1.6 
mils thick, and the clamping ring. The dia¬ 
phragm which is under high tension approach¬ 
ing its elastic limit is very securely held by 16 
screws through the clamping ring into the 
frame. It is assembled under pre-stress by means 
of a special jig and the final tension is then ad¬ 
justed by screwing the button up against it from 
the back to the proper degree until its natural 
frequency is 10,000 c. The diaphragm clears 
the back electrode by only 0.00098 in., this gap 
being determined by special machining and test¬ 
ing methods when the button is made as ex¬ 
plained in detail in Appendix IV. 4 The small 
clearance produces very high diaphragm damp¬ 
ing so that there is no sustention of the natural 
diaphragm frequency and that frequency can 
therefore only be determined by special me¬ 
thods with the electrostatic tester. 4 The gap is 
such that the microphone frequency-response 
curve is extremely flat over the range from 
1,000 to 10,000 c as measured on the electro¬ 
static tester described and discussed in Ap- 





DESCRIPTION AND TECHNICAL INFORMATION 


73 


pendix IV. 4 These characteristics and their 
stability against temperature variations are es¬ 
sential to the success of the aperiodic FEI. 
Matching of thermal expansion coefficients has 
been carefully attended to. Artifical aging of 
the microphones by temperature cycling is an 
important step in their manufacture. 

The active diameter of the diaphragm, de¬ 
fined by the inside diameter of the threaded 
stretching ring, is in. and this conveys an 
idea of the dimensional scale in Figure 25. It 
is an essential feature that the clamping ring 
be very shallow so that in the assembled unit 
no marked acoustic shading or interference 
effects are produced by this ring. Figure 26 



Figure 26. Cross section of aperiodic FEI microphone. 


is a cross section through the assembled micro¬ 
phone and Figure 27 is a cross section through 
the stretching button. 

The space behind the microphone diaphragm 
must have a vent to the outside air to equalize 
changes in pressure accompanying changes in 
altitude. The limits on the time constant of 
this vent are important. Two successful types 
of vent have been used, (1) a fine hole, a mil 
or so in diameter, punctured through the dia¬ 
phragm very near its active periphery; (2) 
a fine scratch made across the profile of the 



Figure 27. Cross section of aperiodic FEI microphone 
stretching button. 


stretching ring where the latter contacts the 
back of the diaphragm. The last type of vent 
seems to be slightly preferable and somewhat 
easier to produce. 

2 6 3 The Aperiodic FEI Transmitter Unit 

The FEI transmitter is of the MOPA type. 
The microphone is mounted directly on top of 
the cylindrical box which shields the oscillator 
components so that a very short internal lead 
wire connects it to the frequency-determining 
elements of its master oscillator. 1 This disposi¬ 
tion can be clearly seen in Figure 28 (as well as 



Figure 28. Aperiodic FEI transmitter unit with two 
hemispherical plastic housings removed. 


in Figure 1). In Figure 28, the two plastic 
hemispherical shell encasements have been 
removed, so that one can see the auto- 

1 The substitution of a cylindrical box for the older 
rectangular box, to shield the r-f transmitter components 
inside the plastic hemispheres, is a minor improvement 
which has been made since Reports OSRD 49 6 7 29 and 
4968 30 were issued. 






























74 


THE ACOUSTIC FIRING ERROR INDICATOR 


matic switch at the right which turns on the 
battery power supply when the target is 
launched. These parts are assembled on a disk¬ 
shaped Dural septum to which the hemispheres 
are attached. One of the assemblies consisting 
of the microphone and MOPA components re¬ 
moved from the housing, is shown in Figure 29. 
Two such assemblies enter the cylindrical shield 
from either end. When the plastic hemispheres 
are assembled on the central septum their out¬ 
side surfaces are practically flush with the 
microphone diaphragms. Wire grills are at¬ 
tached to the hemispheres to protect the dia- 



Figure 29. Microphone and MOPA. 

phragms. The mounting of the transmitter units 
in flags and on gliders has already been described 
and pictured in Figures 1 and 2. 

Two independent end-fed half-wave antenna 
wires are used, one for each of the microphone 
transmitter units. 

In Figure 30 a schematic wiring diagram of 
one of the FEI transmitter-microphone units is 
shown. The oscillator and amplifier triodes are 
in a single tube envelope. The inductively 


coupled coils L! and L 2 of Figure 30 are wound 
on a single plastic core visible in Figure 29 as 
the higher of the two coils. The double triode 



Figure 30. Schematic wiring diagram of FEI trans¬ 
mitter-microphone unit. 

and the antenna output coil L 3 of Figure 30 are 
visible at the bottom of the assembly in Figure 
29. 

2.6.4 physical Characteristics of Aperiodic 
FEI Microphones and Transmitters 

The following list of specifications and phys¬ 
ical characteristics is given to complete the 
information relative to microphone and trans¬ 
mitters. 33 

Microphones 

Diaphragm material. 1.6-mil beryllium cop¬ 
per shim stock rolled “3 numbers hard.” 

Diaphragm (natural) frequency. 10,000 c ± 
300 c. [By diaphragm (natural) frequency is 
meant the frequency at which the diaphragm 
displacement will be 90 degrees out of phase 
with the driving force applied to the diaphragm 
as indicated by the electrostatic microphone 
tester.] 

Active diaphragm diameter. in. 
Diaphragm membrane tension. Approximate¬ 
ly 115 lb per linear inch. 

Diaphragm “Q.” Approximately 0.8. 

Electric capacity of microphone. 25/^f ± 
5^f. 

Air-gap clearance. 0.00098 in. (held by spe¬ 
cial procedure with pneumatic micrometer). 
Air-gap diameter: 0.375 ± 0.002 in. 


































DESCRIPTION AND TECHNICAL INFORMATION 


75 


Dead-air volume. 0.0275 cu in. (annular 
space around back electrode). 

Time constant and vent and dead-air volume. 
Between 0.1 and 0.002 second. 

Relative frequency shift, a/// = 0.025AC/C. 

Sensitivity. AC/CAp, (relative capacity change 
per unit change in pressure) = 2X10 _ti per bar. 
Hence frequency shift A// (/Ap) = 5 X 10 s per bar. 

Microphone frames. Castings of special brass 
alloy as follows: 85 per cent copper; 5 per cent 
tin; 5 per cent zinc; 5 per cent lead described 
as QQ-B-691 Composition 2 casting brass (to 
match thermal-expansion coefficient of berylli¬ 
um copper). 

Microphone clamping rings. Retained with 16 
screws and machined slightly conical to give 
contact on inside edge and spring-washer effect. 

Temperature coefficient of sensitivity. Less 
than 0.015 db per degree centigrade measured 
between 0 C and 60 C. 

Stability of resonant frequency. The resonant 
frequency shall not change more than 1 per cent 
after the microphone has been subjected to a 
temperature cycle from —40 C to +60 C with 
the microphone held for 1 hour at each extreme 
temperature. 

Transmitters 

Radio frequencies. 55.5 and 56.75 me re¬ 
spectively. 

Type of oscillator. Shielded MOPA. 

Modulation. Frequency modulation by con¬ 
denser microphone across tank circuit of oscilla¬ 
tor. 

Tube. Double triode—3A5. 

Coil dimensions. Li—4% turns, No. 12 wire, 
5 turns per inch, %-im diameter; L 2 —5% turns, 
No. 16 wire, 5 turns per inch, %-in. diameter; 
(coils L x and L 2 are inter wound) ; L 3 —4 1/2 turns, 
No. 12 wire, % in. long, % in. diameter. 

Battery supply. 1.5 and 135 v. One Signal 
Corps BA-49 iron-clad battery is used for each 
side of the transmitter in the flag mount. Four 
batteries are used in the glider mount to in¬ 
crease the operating time. 

2 ' 6 ' 5 The Aperiodic FEI Receiving Station 
Model XI-A 37 

Model XI-A, the final model of aperiodic FEI 
receiver defined for manufacturing production 


is shown in Figure 31. It consists of three sepa¬ 
rate chassis assemblies fitting as drawers in a 
single supporting frame. 

This three-drawer model is a minor improve¬ 
ment over the more compact single-chassis 
Model XI. 29 ’ 30 The components and their func¬ 
tions are very similar in the two models, but 
the size of XI-A permits the use of full-size 



Figure 31. Photograph of aperiodic dual r-f FEI receiv¬ 
ing station Model XI-A. 

rather than miniature tubes and affords better 
accessibility for servicing. The upper and low¬ 
er drawers on the right-hand side contain the 
two independent r-f receivers. These convert 
the f-m radio signals from the two units in the 
transmitter into audio-frequency signals de¬ 
lineating the separate responses of the micro¬ 
phones to the impinging N-shaped shock-wave 
excitation. These drawers also contain the audio 
band-pass filter network for each microphone 
channel. The single drawer on the left contains 
the audio-frequency circuits which pulse-length- 
en the shock-wave signals, combine them to 
form the sum signal and the signals indicative 
of directionality and classify the sum signal 
into the three miss-distance zones. 

The block diagram of Figure 32 illustrates 
the functioning of the receiver. Complete 
schematic wiring diagrams are given in Figure 
33A and B. On the block diagram of Figure 32 
the dotted lines separate the contents of the 



76 


THE ACOUSTIC FIRING ERROR INDICATOR 


three different drawers. The two r-f signals 
from the microphone units have frequencies of 
56.75 and 55.5 me, respectively. These are re¬ 
ceived at 1 and amplified in blocks 2 and 3. If it 
is desired to operate more than one FEI system 
in a given locality, other pairs of frequencies 
with similar spacing in this general region can 
be used by slight realignments in the local oscil¬ 
lators of the different receiving stations and 
corresponding changes in the transmitter-oscil¬ 
lator adjustments. The readjustment of trans¬ 
mitter frequency (radio frequency) also implies 
its restandardization for frequency shift in re¬ 
sponse to static pressure on the diaphragm, pro¬ 
cedures normally performed by the manufac¬ 
turer. It is recommended therefore that trans¬ 
mitters adjusted for a different carrier fre¬ 
quency be ordered separately from the factory. 

The antenna cable is provided at 1 with a 
special shielded bifurcation point from which 
two short cables, connected in parallel at 1, lead 
through connectors on the panel to the antenna 
coils of the two receiver channels. The lengths 
of these two short cable leads are carefully pro¬ 
portioned so as to be equivalent to one-quarter 
wavelength. All three cables have a characteris¬ 
tic impedance of 100 ohms and the antenna coil 
which terminates each line is matched to this 
same impedance for its own signal frequency. 
For the signal frequency of the other channel, 
however, the antenna coil presents a terminal 
impedance, Z 0 , of only about 15 ohms. Thus, at 
the junction point, 1, the input impedance, Z i} 
of either channel to the frequency intended for 
the other channel is high, being given by the 
equation relating input and terminal impedances 
for a quarter-wave line, namely, 

Z t = R*/Z 0 . (4) 

As a result, at the junction point, 1, each cable 
accepts r-f signals of the appropriate frequency 
for its channel and rejects r-f signals of the 
frequency of the other channel. The use of this 
cable assembly without modifications in the 
lengths of the bifurcated leads is, therefore, es¬ 
sential to proper functioning. The length of the 
cable from antenna to junction point 1 is imma¬ 
terial within limits. About 50 ft may be used. 

After amplification in blocks 2 and 3 of Fig¬ 
ure 32, the r-f signals are then mixed with local 


oscillator frequencies generated in blocks 4 and 
5, so that at the points 8 and 9 there appear the 
separate and distinct f-m signals for each micro¬ 
phone channel at intermediate carrier frequen¬ 
cies (i-f) of 9 and 10 me respectively. These are 
then amplified in two i-f stages (blocks 10 and 
11). The amplitudes of the i-f signals fed to 
the discriminators for conversion from f-m into 
audio output must be maintained at a very con¬ 
stant level independent of the fluctuations of the 
r-f signal amplitude received at the antenna 
(because of varying transmission distance and 
other conditions affecting r-f signal strength). 
This constancy of i-f amplitude is maintained 
over a wide range of input-amplitude fluctua¬ 
tion, in part by the automatic volume control fed 
back from the limiters, 12 and 13, on the lines 
marked “AVC,” but principally by the (conven¬ 
tional) operation of the two-stage limiters them¬ 
selves. The AVC serves the further function of 
furnishing voltages, indicating the received sig¬ 
nal strengths, which, in the normal position of 
a spring-selector switch (at the point marked 
AVC on the panel of Figure 31) appear on the 
panel meter and indicate the degree of satura¬ 
tion of the limiter. The operator, thus, has a 
means of observing, when the received carrier- 
signal strength is falling, whether it is approach¬ 
ing dangerously near to the lower limit for 
reliable operation. 

The f-m, amplitude-limited signals are con¬ 
verted by the Foster-Seeley type discriminators, 
14 and 15, into low-level audio-frequency sig¬ 
nals at points 16 and 17. The signals at these 
points duplicate electrically the mechanical mo¬ 
tions of the respective FEI transmitter micro¬ 
phone diaphragms in response to the N-shaped 
shock-wave excitation, as indicated by the 
sketched wave profile on the block diagram. It 
will be observed that the discriminator output 
voltage is also used to furnish automatic fre¬ 
quency control [AFC] to the local oscillators, 
so that slight and moderately slow drifts of FEI- 
transmitter frequency are automatically fol¬ 
lowed so as to maintain the i-f carrier fre¬ 
quency in the receiver at the correct constant 
value. (The circuit constants are such that the 
rapid changes in i-f frequency, of which the 
shock-wave signals from the microphones con¬ 
sist, cannot be followed by the AFC. Indeed, 




Figure 

Dashes 


32. Block diagram of receiving station. Typical pulse wave 
— — — separate independent sections of receiving station. 


mes are shown in nulse 














































































































































































✓ 











SULU 



Figure 33A. Schematic wiring diagram for receiver section. 



Figure 33B. Schematic wiring diagram 


for audio and zoning section. 



























































































































































































































































































































































































































































































































































































DESCRIPTION AND TECHNICAL INFORMATION 


77 


the audio frequencies below 200 c, which this 
arrangement suppresses, are far below the 
range excluded by the audio filters 20 and 21.) 

The receiver in use must be first manually 
tuned until it picks up the transmitted signal. 
The AFC can then be turned on to operate auto¬ 
matically. While this initial manual tuning is 
being done the spring selector switch on the 
front panel (Figure 31) is held in the position 
marked “tune” and in this position the AFC 
is automatically disconnected and the panel me¬ 
ter connected to show the discriminator volt¬ 
age. As the tuning button (marked “Freq.” 
on the panel) is exploring across the transmitter 
frequency, the discriminator voltage will pass 
through the familiar positive and negative peaks 
joined by the intermediate linear working range. 
Tuning is correct when the meter indicates the 
zero point at the center of this linear range. The 
spring switch can now be released and allowed 
to return to the normal working position (mark¬ 
ed AVC because, as already explained, the panel 
meter then indicates the AVC voltage). In 
this position the AFC is automatically operat¬ 
ing. A third position marked AFC is provided 
in which the AFC is still operative but the dis¬ 
criminator voltage is thrown on to the panel 
meter in case it is desired to check whether 
the AFC is operating correctly. 

A ready light under the panel meter indicates, 
when lighted, that both channels are operating 

—ma— 1| —nnnnp- 

2 710 JT. c, I87.5mh 
C 2 = 


Figure 34. Diagram of audio band-pass filter, with 
circuit constants. 

correctly with ample radio-signal strength to 
saturate both limiters. In this condition only, 
a relay is closed which permits the tape recorder, 
or the integrating shot counters, to operate. If 
the tuning is faulty, or the received radio sig¬ 
nal strength too low, the light will flicker or 
be extinguished. Such disturbances can oc¬ 
casionally produce spurious shot records which 
must be deducted in scoring. 

The two N-shaped audio-frequency micro¬ 
phone signals at 16 and 17, Figure 32, after 
suitable impedance transformation in the cath¬ 


ode-follower stages 18 and 19, pass through 
the highly important audio band-pass filter net¬ 
works 20 and 21. These filters have carefully 
designed band-pass characteristics which are 
down 3 db from mid-band value at 4,000 and at 
10,000 c respectively, and which have ultimate 



Figure 35. Frequency response of audio band-pass 
filter and equivalent filter circuit. 


slopes on either side of about 12 db per octave. 
The schematic diagram of this filter together 
with specifications for its adjustment appear in 
Figure 34. Figure 35 gives the frequency-re¬ 
sponse curve. This filter eliminates low-fre¬ 
quency noise disturbances but retains the two 
discontinuities, H and T, of the shock wave in 
the form of two transient pulses as indicated in 
Figure 32 at the points 22 and 23. By detailed 
mathematical analysis 30 ” of the transient re¬ 
sponse of this filter it has been shown that the 
higher of the two pips which it gives in response 
to the H and T discontinuities is, to sufficient 
accuracy, proportional to the N-wave peak 
amplitude and independent of the N-wave pe¬ 
riod within the limits of 0.3 to 1.5 milliseconds. 
Figure 36 shows the transient responses of this 
filter to N-waves of different periods as calcu¬ 
lated in the aforementioned analysis. 

The analysis shows that the average of the two pip 
amplitudes would have had even better characteristics in 
this respect, but too many circuit complications seemed 
to be involved in order to utilize this fact. 

The properties of the filter are such that essentially 
only the step discontinuities of the N wave come through 




























































































78 


THE ACOUSTIC FIRING ERROR INDICATOR 



Figure 36. Transient responses of audio band-pass filter to N waves of different periods. 





















DESCRIPTION AND TECHNICAL INFORMATION 


79 


it; the relatively slowly varying intermediate linear 
decline in the N wave, as well as the accidental irregular 
disturbances from noise and other causes, are almost 
completely suppressed. The filter has the further prop¬ 
erty that the pip output which it furnishes in response to 
each input step discontinuity has a peak amplitude nearly 
proportional to the height of the input step and indepen¬ 
dent of the abruptness of rise of the step provided that 
abruptness exceed a certain minimum. This minimum has 
been chosen so that all N waves to be handled by the filter 
are considerably more abrupt than necessary. (The order 
of magnitude of the abruptness of shock-wave discontinu¬ 
ities has been estimated from the theory of their propaga¬ 
tion. See Appendix III. 3 For very intense and hence 
abrupt N wave pressure steps the microphone diaphragm 
is the limiting factor in the abruptness of rise of the tran¬ 
sient input to the filter, however.) The pip amplitude is 
slightly affected by the slope of the linear portion (peri¬ 
od) of the N wave because of the finite time of rise of the 
pip. The H-discontinuity pip is slightly diminished and 
the T-discontinuity pip slightly increased by this effect. 
(This is the reason why the average of the two pips would 
have been a slight improvement.) The filter has further¬ 
more been designed so that the H-transient pip damps out 
rapidly enough to avoid any appreciable superposition 
on the T-pip for N wave periods down to 0.3 millisecond. 
These qualitative statements can be verified quantita¬ 
tively by reference to OSRD 4968, 30b or to Figure 36. 

These transient pips at 22 and 23 do not coin¬ 
cide in time in the two channels because the two 
microphones do not receive the shock wave 
simultaneously. This time displacement, which 
may be of order 1 millisecond, would prevent 
addition of the two microphone signals to form 
the sum response, unless pulse lengthening is 
used. After appropriate audio amplification at 
24 and 25 (with a high-stability voltage gain of 
about 250 through inverse feedback) the pips 
are pulse-lengthened at 26 and 27 in such a way 
that the lengthened pulse exhibits a decay to 
half its initial peak value in 6 milliseconds. At 
points 28 and 29, therefore, the pulse-lengthened 
signals look as sketched on the block diagram. 
The pulse-lengthening consists merely in caus¬ 
ing the voltage pip to charge a condenser 
through a rectifying element. A high-resistance 
shunt leak across the condenser determines the 
above-mentioned subsequent decay rate of the 
pulse-lengthened voltage. Clearly, such an ar¬ 
rangement is noncumulative so that the higher 
of the two pips will dominate in determining 
the pulse amplitude. 

Since the top of the pulse is nearly constant 
for the first millisecond (the decay being only 
about 10 per cent in that time) it is possible to 


combine the signals from the two channels in 
spite of their incomplete simultaneity. This is 
done in the “sum tube,” 30 of Figure 32. The 
sum of the two pulse-lengthened signals appears 
at 31 with shape as indicated in the sketched 
curve S. The highest point reached on this curve 
is the sum-signal amplitude which according to 
its level is selected or rejected by one or more 
of the zoning elements or “flip-flops.” For pur¬ 
poses of scoring it is desirable to classify the 
shots in the three zones of predetermined radii 
and this function is performed by the zoning 
flip-flops. ,n 

A flip-flop is an electronic network which can 
be tripped by a very short impulse to give a 
rectangular plate pulse of fixed duration and 
fixed amplitude. Whenever the input tripping 
pulse exceeds a certain amount, the flip-flop will 
trip independent of the amount by which the 
triggering pulse exceeds the tripping threshold, 
and it will then furnish for a standardized length 
of time a plate signal which can be used on the 
recorder or counter. These networks are de¬ 
signed to have very sharp thresholds. The peak 
value of input pulse at which flipping occurs is 
adjustable by means of bias potentiometers, and 
it is with these that the sizes of the miss-distance 
zones of the target can be set for a given caliber. 
The threshold for tripping is greatest for zone 
1 and least for zone 3. For a very close miss (in 
zone 1) the sum signal is sufficient to trip all 
three flip-flops; for a zone 2, two flip-flops; for a 
zone 3, only the one flip-flop with the lowest 
threshold. 

Two more flip-flops, 35 and 36 in Figure 32, 
are so interconnected that one or the other of 
them trips according to which signal comes first. 
The channel signals must, however, exceed a 
certain bias threshold in order to do this. This 
threshold cannot be set lower than reliable oper¬ 
ation and freedom from accidental disturbances 
permits. It is this threshold setting which fixes 
the maximum lateral sizes of the two direction¬ 
ality lobes of the transmitter. Since it is not 
the sum response which is operative in tripping 
the directional flip-flops, but one of the two in¬ 
dividual channel responses, the lateral extension 

m This method of zoning with flip-flops has replaced the 
earlier method with mechanical relays used in Model XI 
and explained in OSRD 49 67 20 and 4968. 30 For an expla¬ 
nation of flip-flop or trigger circuits see reference 45. 




80 


THE ACOUSTIC FIRING ERROR INDICATOR 


of the directionality lobes must be dependent on 
the orientation of the microphone axis relative 
to the bullet trajectories and on the apex angle 
of the shock-wave cone. The changes in the 
shapes and sizes of the directionality lobes with 
the various parameters on which they depend 
is explained in detail in Appendix II. 2 This 
system in which the directionality-lobe boun¬ 
dary is fixed by the level of the signal from a 
single microphone, the earlier of the two, and 
the later microphone signal is completely ig¬ 
nored, is a marked improvement over earlier 
systems in which the lobe boundary was fixed by 
the difference in level of the two microphone 
signals or by some linear function of the signal 
levels (weighted difference). As a result of 
this improvement, the dimensions of the direc¬ 
tional-response lobes on the two sides of the 
transmitter do not become unbalanced so rapidly 
as the aspect angle departs from zero. The 
limits of correct directional response, however, 
remain unchanged. The choice of directionality 
indication is thus made on the basis of which 
microphone reports its signal first, not as in 
earlier models on the basis of which signal is the 
most intense. A little thought, however, will 
convince the reader that the result is the same. 
If reference is made to Figure 15, it can be seen, 
for example, that within the “reliable range” 
of correct directionality indication the micro¬ 
phone which will receive the shock wave first 
is the one on the same side of the transmitter 
as the trajectory of the bullet. 

The output plug from Model XI-A thus con¬ 
tains a total of five channels, one or more of 
them being activated as the bullet passes 
through one or more of the five regions (three 
miss-distance zones and two directionality 
lobes). Either a tape recorder or a counter can 
be connected to the output terminals. 

Beside the main Model XI-A chassis, a sepa¬ 
rate power-supply chassis is required. 

2 6 6 Specifications of FEI Receivers XI-A 33 
Dimensions 

Receiver. Three-drawer Model XI-A, C2-C, 
Standard Aircraft radio chassis, dust cover 
16x11x16 in., shock mount assembly MT 172-U, 
total weight 40 lb. 

Power supply. B2-C Standard Aircraft radio 


chassis, dust cover 15y 2 xllxlQy 2 in., shock 
mount assembly MT 170-U, total weight 80 lb, 
antenna cable, Twinax cable 95-ohm character¬ 
istic impedance. 

Receiving Channels 

Two channels; one r-f stage in rack; two i-f 
stages in rack; two limiter stages; Foster-Seeley 
type discriminator. 

R-f tuning range: 55.5 me ± 0.25 me (lag 
channel), 56.75 me ± 0.25 me (lead channel). 
(It is also possible to supply 58.25 me db 0.25 me 
and 59.5 db 0.25 me.) 

Tuning: local oscillator only, AFC provided. 

Intermediate frequencies: 10 me (lag chan¬ 
nel) ; 9 me (lead channel). 

Local oscillators: oscillator doubler beloiv 
radio frequencies; 22.75 me (lag channel); 
23.875 me (lead channel). 

Mixer: Inductive mixing and grid leak first 
detector. 

R-f input voltage to saturate limiter at center 
frequency: 40 fiv (approx). 

Impedance at antenna terminals: 95 ohms 
d=40 ohms. 

Discriminator slope: 15 kc ± 3 kc per volt 
(approx). 

I-f band width: flat response to ± 10 per cent 
over 200-kc width. 

Linear range of discriminator: linear to ±3 
per cent over 150 kc. 

Discriminator slope: 15 kc ± 3 kc per volt. 

AFC gain (reduction factor in voltage at the 
discriminator when AFC is connected) : 1:30. 

AFC hold-in range: ±1 me. 

Audio Channels 

A-f band-pass filter: L-section constant K 
confluent band-pass filter as shown in Figure 34 
with frequency-response characteristic shown 
in Figure 35. 

A-f gain (with feedback) beyond filter: 290 
(approx). 

Inverse feedback of a-f amplifier: 10 db 
(approx). 

Frequency flatness of amplifier: ±4 per cent 
from 1 kc to 10 kc. 

Pulse-lengthener time constant: 9 ± 2 milli¬ 
seconds. 

Minimum permissible time between input 
bullet signals: 50 milliseconds. 


nyiNfftrfMTtM 



DESCRIPTION AND TECHNICAL INFORMATION 


81 


Power Supply 

Input supply: 115 v, 60 c or 400 c. 

Output: a, 275 v direct current, 180 ma; b, 
300 v direct current, 30 ma; c, —90 v direct cur¬ 
rent, 5 ma; d, 12.6 v alternating current, 8 amp. 

6 Recording and Indicating Equipment 
for the FEI 

Electrographic Tape Recorder 

This component has been already adequately 
illustrated and described in Section 2.5.4. 

Dial Counters 

The dial-counter chassis assembly has been 
described and pictured in Section 2.5.4. Some 
detailed information regarding the dial counters 
themselves is given here. 

For this application a small, light, compact 
dial impulse counter was needed capable of 
counting random impulses as closely spaced as 
50 milliseconds (twenty counts per second to 
meet the needs of machine-gun fire) and pref¬ 
erably of high input impedance and very low 
power requirement so that it could be driven 
directly from the flip-flop output of the XI-A 
receiver. Specifically, the driving electric pulse 
consists of an (approximately) square wave of 
current of 5-ma peak value and duration about 
8 milliseconds. Each counter chassis assembly 
requires five such dial counters, three for miss- 
distance zone indication, and two for direc¬ 
tionality indications. 

In earlier models Cenco dial impulse coun¬ 
ters had been used but, owing to the low im¬ 
pedance and high power requirement of these 
counters, it was necessary to build a special 
separate power-supply chassis solely for driv¬ 
ing the five counters which when designed to 
meet the Aircraft Radio Laboratory specifica¬ 
tions was rather bulky and weighed over 60 lb. 
This was objectionable especially in the Air 
Force application of the FEI. 

To meet this difficulty, a miniature counter 
was designed in May 1945, and manufacturing 
development was begun with the Doelcam 
Company (West Newton, Mass.) during June 
and July 1945. The general mechanical de¬ 
sign and dimensions are similar to a small dial 
counter with the difference, however, that the 


counting-speed requirement in the present case 
is much lower and the current pulses must be 
of 5-ma peak value instead of 300 ma as in 
Neher’s case. The mechanical parts of the 
counter are built by adapting parts from a 
Waltham stop-watch movement. Figure 37 
shows the external appearance of the counter. 
A main dial is provided on which 100 counts 
are indicated by the larger pointer and a sec¬ 
ondary dial indicates up to 30 revolutions of 
the main dial, so that a total of 3,000 counts 



Figure 37. External appearance of miniature dial 
impulse counter. 

can be recorded. A reset knob, visible in the 
figure, is provided to reset both dials back to 
zero. The frame casting has an outside diameter 
of 2% in. and the dial is about 1% in. in diameter. 
Figure 38 is a view of the counter mechanism 
removed from the case. The electromagnet coils 
(15,000 turns of No. 44 wire, inductance 8.3 
henrys at maximum air gap) have special alloy 
cores which by special heat treatment have a 
permeability,= 50,000 (approximately) in the 
flux density range of 4,000 to 8,000 gausses. 

Insufficient time was available to test and 
develop this counter because the work was 
terminated earlier than had at first been ex¬ 
pected. The first examples of the counter were 





82 


THE ACOUSTIC FIRING ERROR INDICATOR 


received from the manufacturer in September 
1945. Tests of their operation on the square- 
wave 5-ma driving impulses showed all but one 
or two to require more than the specified cur¬ 
rent and they were returned to the factory for 
readjustment of armature-spring stiffness. 
Late in October 1945 five of the readjusted 
counters had been received and tested. After 
some study, satisfactory performance on the 



Figure 38. Internal view of miniature dial impulse 
counter. 


output pulses from Model XI-A receiver at 20 
pulses per second was obtained. It appears that 
one source of difficulty in the action of the 
counter comes from residual magnetism in the 
core material. It has been found that advantage 
can be taken of an inductive reverse surge of 
current at the instant the circuit is broken to 
neutralize this residual magnetization. The 
termination of this work leaves the counter 
development in a less thoroughly completed 
state than could be desired. 

Informing by Lights 

On the panel of the dial counter assembly a 
neon light is provided for each counter to give 
more easily observed indications when hits oc¬ 
cur in the several zones of radial miss distance 
or lobes of directional response. Such lights 
are particularly useful in aircraft applications 
where an instructor operating the FEI receiver 
wishes to give the trainee gunner immediate 
information regarding his successes or errors 
after each burst of his fire or immediately after 
a pass of the target. 


2.6.8 The Quantitative Standardization 
of FEI Response 
Need for Standardization 

The aperiodic FEI differs radically from 
most radio-acoustic devices by being a quan¬ 
titative instrument all of whose elements must 
be standardized so that measurements made 
with them shall have a uniform and reliable 
interpretation for scoring purposes. Without 
this precaution comparative scores made in 
different places, at different times, and with 
different equipment would be meaningless. 

It is here outlined very briefly how this is 
accomplished. Further details as to the instru¬ 
ments and procedures are given in Appendices 
IV 4 and V. 5 

The three main steps which insure that the 
shock wave from a bullet of a given caliber 
(for instance 40 mm) missing the target (by 
4 yd, the smallest zone boundary) will on the 
average give a sum signal in the receiving sta¬ 
tion which will just trip the flip-flop for Zone 1. 
To have this happen, many different adjust¬ 
ments in both the FEI transmitter and re¬ 
ceiver must be correct, but the entire process 
can be summarized by listing the three essen¬ 
tial steps. 

Step One, Transmitter Standardization 

The radio frequency of each f-m transmitter 
must shift by a standardized amount in re¬ 
sponse to a standard pressure (or force) ap¬ 
plied to its microphone diaphragm. This is 
done by the transmitter manufacturer with a 
static pressure applicator of special design 
applied to the microphone diaphragm. A static 
pressure of 40 mm of Hg has been selected as 
the standard value which must produce in each 
FEI transmitter (for both microphone-oscillator 
assemblies) a shift of 100 kc. 

This may also be accomplished by applying 
a standard weight of 50 grams with a bearing 
surface of standard curvature ( 14 ,-in. diameter 
ball bearing) to the center of the microphone 
diaphragm by means of a special jig. The pres¬ 
sure method has been found somewhat prefer¬ 
able and more reproducible, but either method 
can be used. For the FEI microphones, 4 cm of 
mercury pressure produces the same (100-kc) 
shift in transmitter frequency as the 50-gram 


tmrt flJCffuAL 




DESCRIPTION AND TECHNICAL INFORMATION 


83 


weight seated on a contact of y 8 -in. radius of 
curvature at the center of the diaphragm. 

When the weight method of standardization 
is used a small jig, which centers the point of 
application of the 50-gram weight, is placed 
over the machined shoulder on the periphery 
of the microphone. The weight is placed on a 
small platform on the end of a stem fitting 
very smoothly in a lapped bearing in the jig, 
the lower end of the stem having the ball bear¬ 
ing as its termination. 

When the pressure method of standardiza¬ 
tion is used, a small cup provided with a very 
thin rubber membrane over its open end is ap¬ 
plied so that the membrane lies flat on the micro¬ 
phone diaphragm. Air pressure is pumped into 
the cup through a nipple connection and this 
pressure is read with a manometer. 

The standardization procedure then consists 
in adjusting the transmitter trimmer condens¬ 
ers, C x and C 3 (Figure 30) to give correct un¬ 
disturbed carrier frequency (without pressure 
applied to the diaphragm), and correct fre¬ 
quency shift (100 kc) in response to the 
standard pressure on the diaphragm. Since the 
two adjustments are not independent some skill 
and experience is required. The procedure is 
a factory operation. (Condenser C 2 must also 
be adjusted to peak the r-f output.) 

A transmitter standardizing instrument (de¬ 
scribed in Appendix V 5 ) has been developed, 
furnishing a means of checking both the un¬ 
disturbed carrier frequencies in the transmit¬ 
ter and the frequency shifts under diaphragm 
pressure. This instrument uses crystals to 
standardize the frequencies. 

Step Two, Static Firing 

After an FEI transmitter is standardized as 
in step one, measuring the miss distances from 
experimental static firing of the calibers de¬ 
sired with theodolites, a frequency shift vs 
miss-distance curve is obtained for that caliber. 
From this the frequency shift Z 1 for 4 yd, the 
radius of Zone 1 for example can be read. (The 
frequency shift referred to is the sum response 
of the two microphones.) Other zones are cali¬ 
brated similarly. 

Step Three, Receiver Standardization 

The discriminator slope, the audio-frequency 
amplifier gain, and the zoning threshold bias 


of the receiver are to be adjusted so that 
when a sudden measured shift of Z x kc is 
received by special means at the antenna with 
an abruptness simulating the shifts produced 
by the shock-wave discontinuities, the Zone 1 
flip-flop will just respond. Checking equip¬ 
ment for injecting such frequency modulations 
has been developed both for use of the Services 
and for the initial standardization of receivers 
at the factory. This is called an E-checker. Its 
description is given in Appendix V. 5 

It will be noted that steps one and two com¬ 
bined, constitute a determination of peak shock- 
wave pressure and this is a result which should 
be expected to remain constant for a given 
caliber, velocity, and miss distance without 
need for repetition. (To obtain the pressure ele¬ 
vation in the free shock wave from the above 
method it is also necessary to know the reflec¬ 
tion coefficient at the diaphragm and possible 
minor corrections to allow for the obscuring 
effect of the protecting screen over the micro¬ 
phones.) Neither of these two steps considered 
alone, however, has this character. It is there¬ 
fore very important to emphasize that if any 
changes from the present physical design of 
microphone and transmitter are made, these 
may make it necessary to repeat step tivo by 
as extended a program of static firing as has 
already been undertaken in this work. Ap¬ 
parently minor and innocuous changes which 
may even be improvements as far as the func¬ 
tioning of the equipment is concerned must be 
considered carefully from the point of view of 
their effects in necessitating a long and expen¬ 
sive recalibration with static firing (with all 
pertinent calibers, at all requisite miss dis¬ 
tances, aspect angles, etc.). So small a matter 
as the character of the protecting screen over 
the microphone is an example of just such a 
nature. A change from a screen obscuring 5 
per cent of the exposed area to one obscuring 
10 per cent would throw in question the entire 
present calibration status. The same thing is 
true as regards external geometry of the plas¬ 
tic hemispheres and the transmitter mounting. 

It is probably wise to consider that a certain 
amount of occasional checking of the FEI 
transmitters with static fire may always be 
called for, even if no changes in design are 
made. Such checking need not be more fre- 



84 


THE ACOUSTIC FIRING ERROR INDICATOR 


quent than seems indicated by field results, 
and the naturally recurrent desire on the part 
of the Armed Forces to be assured of uni¬ 
formity as to score. 

2 7 BRIEF HISTORICAL REVIEW OF PROJ ECT 

2-71 Early Attempts to Solve Problem 

As already stated, the research and develop¬ 
ment of the FEI did not originate in a Service 
request. The need for such a device was first 
called to the attention of CIT personnel through 
witnessing target practice on airborne targets. 

At the outset the objective of informing the 
gunner as to his errors of fire (if possible 
while the errors were being committed or im¬ 
mediately afterward) was the uppermost con¬ 
sideration. The use of the FEI to score marks¬ 
manship on a quantitative rating scale was an 
application emphasized later by the Armed 
Forces. Secondary interest attached also to 
the question of the direction of the error of 
fire, chiefly for the informing function. This 
is referred to as the indication of direction¬ 
ality. Thus at the outset the problem was re¬ 
garded and approached as a qualitative one 
and emphasis on its quantitative aspects for 
scoring purposes emerged gradually as liaison 
with the Armed Forces became closer. 

The first effort to solve the problem was 
made with magnetized bullets. It was found 
that a .30- or .50-cal. bullet, magnetized before 
shooting, would retain 70 to 80 per cent of its 
magnetic moment when examined after im¬ 
pact in sand. The traveling dipole magnetic 
field was picked up by circular hoop-shaped 
coils of various diameters (1 to 3 ft), at some 
distance from the trajectory, the brief induced 
electric pulse being amplified so as to indicate 
the miss distance by its intensity. At miss dis¬ 
tances of 2 or 3 yd, however, the voltage am¬ 
plitude of the pulse became very low. The law 
of decay of this voltage for a single pickup 
coil is the inverse fourth power of the dis¬ 
tance, because the static dipole field intensity 
diminishes as the inverse cube and the time 
rate of change of this field diminishes as the 
inverse first power of the distance. The in¬ 
duced voltages in such a single pickup coil, 


coming from unavoidable aerodynamic vibra¬ 
tion in the earth’s field incident to towed flight, 
turned out to be of the order of 100,000 times 
larger than the pulses to be detected from .50 
cal. bullets missing the coil by a couple of yards. 
To overcome this, two identical coils with 
windings connected in opposition and spaced 
about one diameter apart were rigidly coupled 
mechanically so that the interference from the 
earth’s field would largely cancel out while the 
nonuniformity of the dipole field of the bullet 
would continue to give a signal. Such a dif¬ 
ferential signal diminishes as the inverse fifth 
power of the miss distance. 

2 7 2 Abandonment of Magnetic Method 
in Favor of Acoustic Method 

In the course of these experiments the acous¬ 
tic shock waves from the passing bullets 
proved to be one of many annoying sources of 
interference, and it was in this way that the 
idea of utilizing the acoustic shock wave for 
the desired objective was first conceived. The 
experiments with magnetized bullets showed 
the magnetic method to be extremely unprom¬ 
ising and fraught with many difficulties, be¬ 
cause of the very low level of intensity of the 
signals picked up and the many sources of in¬ 
terference which are of far greater intensity. 
It was, therefore, abandoned in 1942 at an 
early stage of the work. 

2 7 3 Aperiodic and Resonant Microphones 

The idea of the resonant diaphragms for 
establishing two channels of information be¬ 
tween airborne transmitter and receiver was 
proposed at a very early stage of the acoustic 
experimentation, late in 1942. Field tests and 
laboratory experimentation were at first con¬ 
ducted simultaneously on both resonant and 
aperiodic systems. In the summer of 1943, the 
resonant system appeared from this prelimin¬ 
ary work to be sufficiently promising to war¬ 
rant concentrating every effort exclusively on 
its development. This, however, was before 
the quantitative scoring application of the de¬ 
vice had received so much emphasis from the 
Services. The serious faults in the acoustic- 
response pattern of the resonant system for 



BRIEF HISTORICAL REVIEW OF PROJECT 


85 


scoring did not manifest themselves, however, 
until a great deal of detailed study of the re¬ 
sponse patterns of the dual-microphone res¬ 
onant transmitters had been made by static 
firing. 

Empirical Field Work 

These response patterns were studied by sus¬ 
pending the FEI transmitter from wires at¬ 
tached to a telegraph pole and firing rounds of 
different calibers placed as precisely as possible 
in a large variety of positions relative to the 
transmitter. It was necessary not only to ex¬ 
plore the response for many such positions in 
the target plane and for many transmitter units, 
but also to do so for different orientations of 
each unit relative to the line of fire for dif¬ 
ferent calibers and different ranges from gun- 
to-target. The empirical approach and the dif¬ 
ficulty of interpretation of the response peculi¬ 
arities coupled with the practical difficulties of 
arranging field tests at distant points with the 
use of Army ordnance, aircraft and other 
facilities made progress slow. Nevertheless, de¬ 
tailed field study was absolutely essential and 
indeed it was the results so obtained which em¬ 
phasized the necessity for fundamental re¬ 
search on shock waves and their wave forms, 
without which eventual progress would have 
been impossible. 

This detailed study was necessarily very 
time-consuming for the following reasons: (1) 
It had to be conducted by shooting bullets of 
the various calibers from Army ordnance at 
distant testing ranges with the collaboration 
of the Armed Forces when weather, Army pro¬ 
grams, and facilities permitted. (2) Equip¬ 
ment for measuring the response at the receiv¬ 
ing station had to be progressively developed 
as the requirements became better understood. 
(3) A very great many rounds had to be shot 
partly because: (a) placement of shots was not 
always satisfactory, (b) several rounds must 
be averaged in every case to reduce statistical 
fluctuations, (c) the approach was empirical 
with a great many unknown factors and pos¬ 
sibilities to be tried and explored. 

In addition to the static firing tests it was 
necessary to conduct many tests of the trans¬ 
mitters in actual towed flight in sleeves and 


flags because it was soon found that mechani¬ 
cal vibration, wind, and flapping noise and 
other interferences were problems calling for 
extensive study. In addition, the reliable per¬ 
formance of new transmitter unit designs un¬ 
der the severe treatment they received in 
towed flight tests had to be carefully checked 
by trial. Large numbers of transmitters had 
to be constructed for such tests since from 
four to eight were frequently lost in a single 
mission. Such tests had to be conducted with 
Army tow planes when weather permitted and 
the planes and personnel could be spared from 
regular duty. A certain number of such tests 
were conducted with actual firing of guns at 
the towed transmitter when this could be ar¬ 
ranged. 

Studies of the types described continuing 
through the year 1943 made it increasingly 
clear that the earlier optimism regarding the 
resonant system had been unjustified. It was 
found to have two major faults. (1) The res¬ 
onant diaphragms were found to be very sensi¬ 
tive to the mechanical vibrations arising in 
towed flight, and to the characteristic noises 
connected therewith. (2) The sum-response 
patterns turned out to be not only very irreg¬ 
ular in shape but, as already pointed out, the 
miss distance was not a single-valued function 
of sum response. For a given resonant trans¬ 
mitter, critical null response regions in the 
target plane existed, while at greater and at 
lesser radial miss distances, the sum response 
was well-defined. A given level of sum response 
might thus mean any one of several widely 
different miss distances. The difference lobes 
suffered from similar peculiarities. 

These facts alone are sufficient to arouse the 
suspicion that the wave form of the shock 
wave has some sort of definite period which 
resonates constructively and destructively with 
the natural periods of the resonant micro¬ 
phones, and this was found to be the case 
after sufficient fundamental study on the na¬ 
ture, wave forms, and laws of propagation 
of ballistic shock waves had revealed the facts. 

For a complete analysis and discussion of 
all the sources of error for the cases of both 
the resonant and aperiodic systems the reader 
is referred to OSRD Report 4966. 22 







THE ACOUSTIC FIRING ERROR INDICATOR 


86 


2-75 Fundamental Research on the Physics 
of Ballistic Shock Waves 

The difficulties encountered with the res¬ 
onant system of firing error indication led, 
in October 1943, to initiation of an extensive 
program of fundamental research on the phy¬ 
sics of ballistic shock waves, upon which the 
present design of aperiodic FEI is based. Space 
in this report permits mentioning only a few 
of the most important results. For more com¬ 
plete information, the reader is referred to 
Appendix III. 3 

Special oscillographic equipment was devel¬ 
oped for recording wave forms of shock waves 
as observed with different microphones. A quartz 
piezoelectric microphone of extremely high 
period was obtained from the Bell Telephone 
Laboratories. The responses to shock-wave exci¬ 
tation of the resonant condenser microphones 
and also of aperiodic condenser microphones 
were also studied. It was found that the shock- 
wave pressure disturbance has an N-shaped pro¬ 
file, already alluded to. The very steep pressure 
steps in the wave and the variation in period 
(measured between the occurrence of these head 
and tail discontinuities) were striking features 
of this research. It was decided to utilize profit¬ 
ably the steep pressure steps in such a way as to 
differentiate the shock-wave signals from ordi¬ 
nary sounds. Very excellent direct photographs of 
shock waves made at Aberdeen Proving Ground, 
were sent from there to this project and a study 
of these was of the greatest help in interpreting 
the acoustic wave forms. Much insight into the 
mechanism of propagation of the discontinui¬ 
ties was gained from an article by R. Becker, 44 
and it was in this way that a quantitative 
idea of their extreme steepness was formed, 
which gave reason to expect the presence of 
very high audio-frequency components in the 
shock-wave spectrum even at large distances 
from the bullet trajectory. The idea of using 
this fact to avoid noise interference by limit¬ 
ing the audio-frequency spectrum employed 
with a band-pass filter which would (1) 
eliminate noise, and (2) transmit the height 
of the N-wave discontinuity step as a trans¬ 
ient pulse independent (within limits) of the 
N-wave period was at first only a “pious hope.” 
It was not certain (1) that the signal-to-noise 


ratio could in this way be improved by filtra¬ 
tion, and (2) that a filter having the required 
response to the transient excitation could be 
designed. A third uncertainty concerned the 
possibility of designing a microphone with 
flat frequency-response characteristics up to 
a sufficiently high frequency to take advantage 
of this plan of attack. The use of a flat-re¬ 
sponse microphone seemed attractive never¬ 
theless since these components must have re¬ 
producible characteristics in a very large num¬ 
ber of examples and a more complicated re¬ 
sponse curve would be more difficult both to 
specify and to reproduce. Also, it was soon 
realized that a flat-response microphone per¬ 
mitted a simple static standardization proce¬ 
dure (by the pressure, or weight, method) 
since the response over the working range 
becomes identical to the response at zero fre¬ 
quency. 

2-76 The Aperiodic System of Firing 
Error Indication 

Work on the aperiodic system, both along 
theoretical lines and in the field and labora¬ 
tory, was started intensively early in the 
spring of 1944. It was not until late July of 
that year that the improvement in signal-to- 
noise ratio of the band-pass filter was verified 
(4,000 to 10,000 c). 26 This was a very impor¬ 
tant result for the success of the device. Much 
time and effort had been spent in mathematical 
analysis on this filter design and on the calcula¬ 
tion of its response to the N-wave transient. 301 * 

Problems concerned with the technique of 
reproducible microphone manufacture and sta¬ 
bility occupied a large place in this work 
throughout its entire duration and efforts 
tending toward improvements along this line 
continued almost up to termination of the work. 
A complete history of all the phases of this prob¬ 
lem, in the evolution to the present manufactur¬ 
ing procedure, would be too long for this report. 
The reader is referred to Interim Progress 
Reports 8 - 9 - 10 - 1214 ' 17 - 19 - 20 ' 21 ’ 23 - 26 ' 27 - 28 ' 31 - 34 for such in¬ 
formation. 

Actual validation of the scoring accuracy of 
the aperiodic FEI in towed flight did not come 
until very late (May and June 1945). This 
work, which required special camera equip- 



BRIEF HISTORICAL REVIEW OF PROJECT 


87 


ment and close cooperation with the Armed 
Forces was done at Ft. Bliss, Texas. 32 

The work for the Manhattan District in the 
application of the FEI to measurement of 
blast pressures from the atomic bomb was 


started in April 1945, and cooperation to that 
end continued until the termination of World 
War II. 

Research and development on the FEI under 
NDRC terminated on October 31, 1945. 



Chapter 3 


MAGNETIC RECORDING RESEARCH 

By George E. Beggs, Jr:' 


3i SUMMARY 

HIS REPORT DESCRIBES results of work on the 
two important phases of magnetic recording 
research, the development of recording methods 
and apparatus, and of magnetic signal carriers' 5 
of various types. The work was done under a con¬ 
tract with the Brush Development Company. 
Portions of the work on magnetic recording 
media were done under subcontracts with Bat- 
telle Memorial Institute and Case School of 
Applied Science. 

A magnetic transient recorder [MTR] is de- 
cribed, having a frequency range extending 
from about 800 to 30,000 c, over which phase 
distortions are held to a minimum by special ap¬ 
plication techniques, also described. 

The development of recording techniques in¬ 
troduced new problems in the field of record¬ 
ing materials. At the initiation of the program, 
available materials were not fully satisfactory 
for applications at hand, both from the fre¬ 
quency-response and the signal-to-noise ratio 
characteristics. Materials and methods for pro¬ 
ducing them were studied. Two were developed 
which have considerable merit, electroplated 
ribbon [ER], and a powder-coated tape [PCT]. 

The first type was produced by electroplat¬ 
ing a thin layer of a magnetizable alloy onto 
a metallic, nonmagnetic base. The majority of 
the numerous platings developed consisted of 
cobalt-nickel alloys of controlled magnetic 
properties on a fine ribbon base of phosphor 
bronze. 

The second consisted of a tape of either 
paper or plastic material, coated with a thin 
layer of a rather concentrated dispersion of a 
fine magnetic powder in a lacquer-type binder. 
From a number of powders investigated, syn¬ 
thetic magnetite (Fe ;i 0 4 ) was finally chosen 
because of its favorable magnetic and mech¬ 
anical properties. 

a Technical Aide, Section 17.1-17.2, NDRC. 

b SC-111 


Both types of signal carriers exhibit fre¬ 
quency responses superior to hitherto commer¬ 
cially available recording media. 

In connection with the application of this 
material, or modified forms of it, to magnetic 
recording, a new ring head [RH] was devel¬ 
oped for recording and reproducing. It has re¬ 
duced iron losses and an improved frequency 
response, is inherently hum-free, and can be 
produced in quantities because of its mechani¬ 
cal simplicity. 

As a final result of the work under this pro¬ 
gram, the ground work for a theory of the 
mechanism of the magnetic recording and re¬ 
producing process was laid. The ultimate goal 
of this theory is the quantitative correlation 
between the performance of a magnetic signal 
carrier on the one hand, and its magnetic and 
mechanical properties on the other. Sufficient 
time was not available to complete work along 
these lines, but sufficient information was 
gained to allow the qualitative prediction of 
the performance of a given signal carrier. 

3.2 INTRODUCTION 

Ever since the recording of intelligence at¬ 
tained practical importance, the demand has 
existed for decreasing the amount of signal 
carrier required to store a given amount of 
information. This striving for economy has 
been apparent in all three dominant systems 
of signal recording; in the mechanical, the 
photographic, and the magnetic. Particularly 
in the latter field the trend is still in rapid 
evolution. 

The principle of recording sound patterns 
upon a record of steel wire, tape, or similar 
materials has been known for many years. 
Valdemar Poulsen, a Danish engineer, intro¬ 
duced the method over forty years ago, and 
demonstrated his Telegraphone at the Paris 
Exposition in 1900. 

The magnetic system of recording offers cer- 



88 



THEORY 


89 


tain advantages over other methods in several 
respects, but particularly in applications where 
records of a temporary nature are desired, in¬ 
asmuch as the same recording material may 
be used over and over again to make new re¬ 
cordings after the old ones have served their 
purpose. In such instances, the carrier may 
be cleared (erased) and made ready for reuse 
by simply passing it through a magnetic field 
of appropriate characteristics, either alternat¬ 
ing current or direct current, just prior to re¬ 
cording a new intelligence signal. Continuous 
recordings of long duration may be made upon 
a single carrier, thereby avoiding the neces¬ 
sity for multiple recorders or interruptions to 
change records. Furthermore, a magnetic 
recording may be reproduced hundreds of 
times without appreciable loss in signal level 
or quality. 

The original problem considered under this 
program was the development of a high-fre¬ 
quency MTR to make a transient record for 
later continuous repetitious reproduction to 
permit observation on a cathode-ray oscillo¬ 
scope. Magnetic recording was selected to al¬ 
leviate the problem of multiple reproductions, 
ease of operation, small size, and reasonable 
power requirements. Early in this portion of 
the program, it became apparent that improve¬ 
ments in recording materials were needed to 
allow adequate frequency response of the sys¬ 
tem at usable signal-to-noise ratios. Thus the 
second portion of the program was concerned 
with the development of an improved magnetic 
signal carrier which would have these two 
qualities and an increased economy in the total 
required carrier volume for a given amount 
of intelligence. 

In order to reduce this carrier volume, both 
its cross section and its velocity in recording 
may be reduced. The cross section of a carrier 
is mainly dictated by mechanical factors such 
as strength, and wearing and handling prop¬ 
erties. Though a thin carrier is also desirable 
for performance reasons, there is apparently 
little room for the theoretical prediction of a 
minimum carrier cross section. Most of the 
work was concerned with the theoretical and 
practical aspects of improving the magnetic 
properties of the signal carrier. 


33 THEORY 

Elementary Considerations 

Certain comments on response characteris¬ 
tics to be expected from magnetic recording 
processes will prove helpful in interpreting 
developments described later. 

In the simplest approach, the recording flux 
will impress upon each point of the passing 
tape of magnetic induction which corresponds 
to the different instantaneous signal levels. 
This impressed flux may be called the internal 
flux [IF]. With longitudinal magnetization, 
used in the majority of magnetic recording 
studies, the external flux, as picked up by the 
recording head, must be proportional to d<f>/dl, 
where <f> is the internal flux, and l is the dis¬ 
tance along the tape. In other words, the 
external flux must be inversely proportional 
to the wavelength recorded, and therefore 
should increase 6 db per octave for constant 
recording. In playback, the voltage generated in 
the reproducing-head coils depends on the rate of 
change of flux. Therefore, one might expect a 
12 db per octave increase in output voltage, 
and a phase shift of 180 degrees. This be¬ 
havior can actually be observed, but only for 
very low frequencies, i.e., wavelengths about 
1 in. or longer. For somewhat shorter wave¬ 
lengths the rise has been found to be about 
6 db per octave. The reason for this apparent 
discrepancy lies in an oversimplification of the 
preceding considerations. It appears that the 
flux through the reproducing head is deter¬ 
mined not only by the magnetic state of the 
carrier element in actual contact with the 
head, but also by the adjacent elements on 
either side of it. The overall effect is one of 
adding or integrating the effects of the ad¬ 
jacent carrier elements. This integration of 
the second derivative (playback voltage de¬ 
pendent upon rate of change of flux) results in 
a playback voltage proportional to the first 
derivative of the signal, under constant cur¬ 
rent recording conditions, or a rise of 6 db 
per octave. For very low frequencies (long 
wavelengths), the effect of the integration is 
small, because of the rapidly decreasing in¬ 
fluence of the adjacent elements with distance, 
which accounts for the observed rise of 12 db 





90 


MAGNETIC RECORDING RESEARCH 


per octave as explained by the oversimplified 
discussion. 

With rising recording frequency, the output 
voltage should keep on rising with a 6 db 
per octave slope, if it were not for two addi¬ 
tional effects which become more and more 


\ 

2 



A 

THICKNESS UNIFORMLY MAGNETIZED 


THICKNESS OF 
SIGNAL CARRIER 



DEPTH OF 
FLUX PENETRATION 


THICKNESS PARTIALLY MAGNETIZED 


Figure 1. Penetration of recording flux into signal 
carrier at (A) long wavelength, (B) short wavelength. 


dominant. These are the effects of demagne¬ 
tization of the elementary “bar magnets,” and 
that of limited penetration. In this connection 
see Figures 1 and 2. 

Demagnetization 

The demagnetization of magnets, that is, the 
weakening of their external field strength 
caused by their own magnetization, has been 
extensively treated in the literature, in par¬ 
ticular for straight cylindrical rods. However, 
the immediate quantitative application of 
available data to magnetic recording problems 
is difficult for two reasons. (1) The elementary 


magnetization of the elementary magnets, for 
higher frequencies, is not uniform throughout 
the carrier cross section except for extremely 
thin carriers. This fact reduces the effective 
cross section of the elementary magnets and 
will therefore decrease their demagnetizing 
factors compared to those for full cross sec¬ 
tion. The situation becomes further compli¬ 
cated by some shunting effect of the remain¬ 
ing nonmagnetized portion of the carrier. 

Coercive Force and Remanence 

The determination of the flux available from 
a magnet is well known from the design of 
permanent magnets. Some discussion of facts 
derived from this design data will serve to 
clarify the discussion above, and will illustrate 
the possibility of assigning figures of merit to 
recording media. 

In the second quadrant of the hysteresis 
loop (see Figure 3) containing the so-called 




Figure 2. Arrangement of elementary bar magnets in 
signal carrier with sinusoidal signal recorded. 

magnets in a magnetic sound carrier are linked 
together with their ends of like polarity ad¬ 
jacent, whereas the published data refer to 
magnets with free ends. The demagnetizing 
factors are consequently different in the two 
cases. (2) Because of limited penetration, the 


demagnetization curve, a straight line (shear¬ 
ing line) is drawn from the origin at an angle 
a with the B axis. Tangent a equals the demag¬ 
netizing factor of the magnet. The point of inter¬ 
section of the shearing line and the demagnetiza¬ 
tion curve defines the working point of the 
magnet, the ordinate of this point describing the 
available magnetic induction, or flux. 

For bar magnets with a very high ratio of 
length to diameter, i.e., for long wavelengths 
and very small demagnetization, the shearing 
line is practically vertical and the available 
induction, consequently, identical with the 
remanent induction of the material. As long as 
the available induction remains close to this 
point, the response curve will follow very near¬ 
ly the rising trend of 6 db per octave. With de- 

































THEORY 


91 


creasing wavelength (increasing frequency) 
the demagnetizing factor increases and the 
working point slides down the demagnetiza¬ 
tion curve as the slope of the shearing line de¬ 
creases. Accordingly, less and less induction 
becomes available for the generation of the 
playback voltage and the rising slope of the 
response curve decreases steadily. 

The rate at which the available reproducing 
flux decreases depends—for a carrier of given 
dimensions and velocity—on the shape of the 
demagnetization curve. The greater its slope 
in any one point, the greater will be the loss 
increase in the corresponding point of the 
frequency-response curve. The correlation be¬ 
tween these two quantities allows actual der¬ 
ivation of the response curve. In general, this 
derivation can only be made by graphic means, 
since accurate analytical expressions for either 
the demagnetization curve or the demagnetiz¬ 
ing factors do not exist. The effects on the 
response curve can be visualized by reference 
to Figure 4. 

A rather rough but quite useful rule can be 
derived from these considerations. Since the 
ratio of coercive force to remanence, H c /B r 
gives the reciprocal of the average slope of 
the demagnetization curve, this ratio can be 
used as a figure of merit expressing the suit¬ 
ability of the material for magnetic recording 
purposes. The greater the ratio, the smaller 
will be the rate at which the response curve 
falls off in the upper frequency range, for given 
physical dimensions and carrier velocity. While 
this figure of merit does not give fine details on 
response characteristics, it does give an idea of 
general performance to be expected. The rule 
has been tested and verified in many cases. 

Penetration Effect 

The penetration effect, mentioned above, 
further limits the high-frequency response, at 
least when RH’s are used. Whether this effect, 
or the effective gap width, or demagnetization 
has the greatest limiting effect on the higher 
frequencies would be of interest in comparing 
plated vs homogeneous materials. Time did 
not allow these comparisons. The penetration 
of the high-frequency flux into the recording 
medium is limited because of the magnetic 


skin effect, and because of the geometry of 
flux distribution within the medium. The first 
effect is well known and needs no further ex¬ 
planation. The second one becomes clear from 
the fact that the flux from the RH enters into 


EFFECT OF 



Figure 4. Evolution of frequency-response curve. 


the recording medium at a small angle (de¬ 
pending, primarily on the magnetic properties 
of this medium) which makes full penetration 
of an appreciable thickness impossible at short 
wavelengths. (See Figure 1.) The wavelength 
effect appears to predominate under most 
conditions. Thus a further decrease in high- 
frequency response is noted, as illustrated in 
Figure 4. 

An experimental proof of this penetration 
effect is the fact that the erasing with high 
frequency of a record on wire or tape, other 
conditions being equal, becomes increasingly 
more difficult as the recorded frequency de¬ 
creases ; i.e., the lower frequencies penetrate 
deeper into the recording medium than the 
higher ones, which may fall into the region 
that can be reasonably well penetrated by the 
erasing frequency. 

A further evidence of limited high-frequency 
penetration is the effect from twisting of a 
round wire. For a given wire the level of the 
playback voltage decreases as the recorded 
frequency and twist angle between recording 
and reproducing heads increase. (See Figure 5.) 

The above discussion outlines qualitative 
bases for the general form of response curve 
to be expected, as shown in Figure 4. 







92 


MAGNETIC RECORDING RESEARCH 


34 MAGNETIC TRANSIENT RECORDER 

Military Requirements 

The contractor was originally requested to 
develop an MTR capable of recording and re¬ 
producing transients containing frequencies 
in the range from 300 to 30,000 c, with good 
signal-to-noise ratio, low phase distortion, and 
provision for continuous reproduction of the 
recorded signal to allow its observation on a 
cathode-ray oscillograph. Total recording time 
of the order of 25 to 50 milliseconds was de¬ 
sired. 


development, it was apparent that new tech¬ 
niques would need to be studied, to allow the 
higher frequencies to be successfully recorded, 
and to bring phase distortion to a minimum. 

Initial tests on a high-frequency recording 
system were made, utilizing a tape about 3x120 
mils, at a velocity of 35 ft per second. This 
velocity was an arbitrary choice, higher than 
expected for final apparatus, to allow partial 
elimination of this variable in studies on the 
effect of pole-piece materials, signal-to-noise 
ratio, and frequency response. The initial tape 
used was a tungsten steel alloy. 



AXIAL ANGULAR DISPLACEMENT BETWEEN RECORDING AND REPRODUCING HEAD IN DEGREES 


IOOO C(X*60 MILS) 
2000(30) 

3000(20) 

4000(15) 

5000(12) 

7000(8.6) 

10,000 ( 6 ) 


Figure 5. Effect of wire twisting upon playback voltage (experimental evidence of limited magnetic penetration). 


General Methods Studied 

Prior to the beginning of work on the de¬ 
velopment of the MTR, some work had been 
done in this field on a unit capable of record¬ 
ing transients with frequency components 
from direct current to 500 c. 1:{ In this instru¬ 
ment the transient is recorded on a loop of 
magnetic tape and played back synchronously 
every 0.2 second for observation on an oscil¬ 
loscope screen. In connection with this work, 
the problem of phase distortion in magnetic 
recording was brought to the fore. However, 
in this particular case, since very low fre¬ 
quencies were to be recorded, while the high- 
frequency response was restricted, a carrier- 
frequency system was used, minimizing the 
phase-distortion problem. The carrier fre¬ 
quency was amplitude modulated by the in¬ 
telligence frequencies. 

With the advent of requirements for a re¬ 
corder capable of handling frequencies up to 
30,000 c, more than ten times the carrier fre¬ 
quency of 2,200 c used in the above-mentioned 


The actual recording method consisted of 
main flux longitudinal recording, using two 
flat poles, slightly offset, on opposite sides of 
the tape, as shown in Figure 6. In these tests 
obliteration and biasing were used. Results 
showed that signal-to-noise ratios were not 
satisfactory for the desired frequency-response 
range of 300 to 30,000 c, although actual re¬ 
sponse characteristics were encouraging. 

Interesting data were obtained on the num¬ 
ber of reproductions possible from a magnetic 
recording. Figure 7 shows the decay of signal 
as a function of the number of reproductions. 
Although these data were based on early experi¬ 
mental evidence, they serve to confirm state¬ 
ments made earlier concerning the number of 
reproductions available from magnetic record¬ 
ing media, one of the reasons this particular 
system was chosen for a transient analyzer. 

Recording Methods 

It is desirable to give a brief description of 
the two methods of recording that have been 
widely used in the field to explain the terms 






































MAGNETIC TRANSIENT RECORDER 


93 


d-c obliteration, d-c biasing, a-c biasing, etc. 
Prior to recording it is necessary for the tape 
to be placed in a uniform magnetic state. This 
may be a partially saturated state, obtained as 



Figure 6. Pole and flux configuration for longitudinal 
recording—early method. 


a result of saturation of the tape when passing 
a d-c obliterating magnetic circuit, or a neutral 
state, which can be obtained by using a-c ob¬ 
literation. 

If d-c obliteration is used, the tape leaves 
the obliterating head in a condition deter¬ 
mined by the magnetic remanence after satura¬ 
tion. To allow proper recording of the a-c in¬ 
telligence signal following the obliteration 



Figure 7. Decrease of signal versus number of re¬ 
productions. 


process, it is necessary to superimpose the signal 
current on a d-c polarizing current, or bias cur¬ 
rent, to produce a unidirectional pulsating mag¬ 
netizing force between the pole pieces so directed 
that it has a tendency to demagnetize the tape 
from its remanent magnetic state. This system 


of recording thus employs d-c obliteration and 
d-c biasing (pure d-c system). 

Where a-c obliteration and a-c biasing (pure 
a-c system) are used, high-frequency currents 
are supplied to the obliterating head as the 
tape passes. A-c obliteration should leave the 
tape or wire in its neutral or demagnetized 
state. To achieve this, a field with a peak 
value sufficient to saturate the carrier, and 
gradually decaying along the carrier axis, 
should be generated. In practice a normal re¬ 
cording head is utilized as an obliterating head, 
but the field decay along the axis is too abrupt. 
Consequently, some recording of the obliterat¬ 
ing frequency occurs, but the amplitude of this 
recording may be kept very small by choosing 
an obliterating frequency whose recorded wave¬ 
length is smaller than the effective gap width 
of the recording head. This method has proved 
quite satisfactory for most practical applications. 

Gap Width 

Gap width and effective gap width are terms 
peculiar to magnetic recording. The actual gap 
width of a given magnetic recording head is 
the physical distance between the edges of the 
two pole pieces, measured parallel to the car¬ 
rier axis. The effective gap width is larger 
than this, and is determined by the field dis¬ 
tribution over an incremental length of the car¬ 
rier influenced by the recording head at any 
particular instant of time. Ideally, the field 
distribution over the length of the increment 
should be uniform, and sharply defined, i.e., it 
should be rectangular in shape, and limited to 
the dimensions of the actual gap noted above. 
Because of magnetic leakage the shape is never 
rectangular, but decreases gradually on either 
side of a maximum. The width of the cor¬ 
responding “effective” rectangle is somewhat 
larger than the actual gap width, and repre¬ 
sents the effective gap width. 

The d-c biasing and obliterating method de¬ 
scribed allows recording on the essentially 
straight portion of the hysteresis loop of the 
carrier material. A-c biasing with a-c obliterat¬ 
ing utilizes the artificially straightened center 
portion of the normal magnetization curve. 17 
The a-c bias and obliterating system is some¬ 
what similar to the effect obtained in the op¬ 
eration of a push-pull amplifier, wherein the 


































94 


MAGNETIC RECORDING RESEARCH 


curved tube characteristics are combined in 
the positive and negative excursions of the sig¬ 
nal to produce an overall straight line transfer 
characteristic between the applied grid signal 
and the output signal. In the magnetic case, 



Figure 8. Determination of correct bias field and 
construction of overall magnetic transfer characteristic. 


the positive and negative portions of the nor¬ 
mal magnetization curve are shifted in the 
proper directions by the applied bias field, so 
that the signal superimposed on this bias field 
(not modulating it, but adding to it) sees the 
transfer characteristic to the flux distribution 
in the carrier as a straight line rather than a 
curved line. (See Figure 8.) 

Either of the two methods leads to essentially 
similar frequency-response curves; the a-c 
method, however, offers a higher signal-to-noise 
ratio, usually about 5 to 8 db better, because 
the neutralized signal carrier, theoretically, 
cannot generate any noise voltage in the re¬ 
producing head. 

It is also possible to combine the pure d-c or 
pure a-c methods, by using d-c obliteration and 
a-c biasing, either adding the signal to the a-c 
bias, or using the signal to modulate the a-c 
bias. The magnetic status of the carrier after 


it leaves the recording head will correspond to 
the crest value of one envelope of the bias-plus- 
signal signal, the other envelope merely tend¬ 
ing to saturate further an already saturated 
carrier. Again, these conditions obtain when 
the effective gap width is greater than the 
biasing wavelength. Figure 9 illustrates the 
mechanism explained. This particular method 
has not been widely used, as the pure a-c method 
offers signal-to-noise ratio advantages, while the 
pure d-c method offers simplicity advantages. 

Recording Materials 

The initial development work on the MTR 
indicated that the use of an endless carrier 
was desirable. This meant that the ends of 
the tape or wire had to be joined. Such a 
joint produces a magnetic discontinuity, which 
may produce a signal which will completely 
obliterate, or at least seriously distort, the tran¬ 
sient being studied. It therefore appeared de¬ 
sirable to investigate the possibility of plating 



Figure 9. A-c bias, d-c obliteration conditions. 


a nonmagnetic base with a magnetic layer 
to serve as the actual signal carrier, such 
plating, or coating, to be applied after the 
joining of the carrier, to reduce or eliminate 
the effect of the joint. Although this idea ap¬ 
peared feasible, it was not incorporated in the 
MTR as finally constructed, since plating tech¬ 
nique, and some factors governing magnetic 


^ONFirntr I'fTT* 

































































MAGNETIC TRANSIENT RECORDER 


95 


behavior of platings, had not been worked out 
at the time. The initial work on platings in¬ 
dicated that other advantages could be ex¬ 
pected from a thin magnetic layer beside the 
elimination of joints, etc. Thus, as described 
later in this report, considerable work was done 
on the development of new magnetic recording 
media along the lines of thin magnetic layers 
produced by various means. 

Magnetic Transient Recorder 

The MTR utilized a highly polished steel 
tape, about 3x120 mils, operating at a speed 
of 35 ft per second, giving a total recording 
time of 20 to 40 milliseconds, with the length 
of tape supplied. Signal-to-noise ratios of the 
order of 35 db were obtained, with frequency 
response from 800 to 30,000 c. Phase shift was 
held to a minimum by an interesting process 
described later in this report. The response 
and signal-to-noise ratios were obtained by the 
use of the pure a-c system, proper tape speeds, 
recording- and reproducing-head gap widths, 
and proper head materials and assemblies. It 
was found that two 0.007-in. transformer iron 
strips utilized as a laminated recording head, 
with two dimensionally similar Mu-metal strips 
as a reproducing head, gave good results. Al¬ 
though other types of heads, known as ring heads 
(see Section 3.5.2) were developed during the 
investigation, primarily for use with plated ma¬ 
terials, they were not used in this particular 
instrument, sufficient data not having been accu¬ 
mulated at the time. 

Phase-Shift Elimination 

Phase-shift elimination was accomplished by 
a re-recording process. Though it may be pos¬ 
sible to design a network which will provide 
a phase-shift compensation without serious 
amplitude distortion, the re-recording method 
described below is considerably simpler, and 
serves the purpose. 

Let us assume that the recording head and 
the recording amplifier form a unity having 
an unknown time delay L which is a function 
of frequency and that the reproducing head 
and its associated amplifier have a time delay 
N which is also a function of frequency. The 
overall time delay between the input terminals 
and the output terminals of the equipment is 


then given by the sum L -\- N + T. T is the time 
delay introduced by the distance between the 
recording and reproducing heads, which is in¬ 
dependent of frequency and therefore can be 
neglected. If, after recording, the direction of 
the carrier motion is reversed, for playback, 
the overall time delay will be N — L. The minus 
sign associated with L comes about since the 
most delayed portions of the signal become 
the most advanced due to the change of direc¬ 
tion. The signal derived from the reproducing 
head is now backwards. By re-recording this 
signal obtained from the reversed carrier on 
to another carrier through the identical ampli¬ 
fier and recording head, we obtain on the sec¬ 
ond carrier recording with time delay N, 

(N — L) + L = N 

First recording Second 

and playback recording 

If this re-recorded carrier is reversed, we ob¬ 
tain a time delay N — N = 0 at the output termi¬ 
nals, and the signal is no longer backwards. 

Oscillographic proofs of this method are con¬ 
tained in Figure 10. Part 1 shows the signal 
at the input terminals of the recording ampli¬ 
fier. Part 2 shows the signal at the recording 
head. Phase shift occasioned by the amplifier 
and head is present. Part 3 shows the repro¬ 
duced signal after reversal of the carrier, as 
seen at the output terminals of the reproducing 
amplifier. Part 4 shows the recording-head sig¬ 
nal while making the second recording. Part 5 
is the signal obtained from the reproducing 
amplifier after the second and final carrier re¬ 
versal. Without the phase-shift compensation 
method employed, the signal shown in Part 6 
is obtained. Some low-frequency disturbances 
obscure the results somewhat, but it can be 
seen that almost complete phase-shift cancella¬ 
tion has been effected. Re-recording has one 
disadvantage, in that it reduces the overall 
signal-to-noise ratio which can be realized. 
Otherwise, it appears to be satisfactory as a 
means of phase compensation. 

MTR Construction Details 

The MTR as finally constructed is briefly de¬ 
scribed below, including the phase-shift cancel¬ 
lation method outlined above. At the time of 
the construction of this unit it was apparent 
that considerable additional work would have 


tlt|>iiiiiiL|||hIT [ r| | 

- -v rmhi 



96 


MAGNETIC RECORDING RESEARCH 


to be done to improve characteristics of re¬ 
cording media then available. While such work 
was being carried on, it was decided to con¬ 
struct at the same time a unit using a steel 
tape, without plating but with soldered or 
welded joints to allow the use of a continuous 


loop. The basic configuration of the instrument 
circuit is shown in Figure 11. Since a re-record¬ 
ing process is necessary to eliminate phase 
shift, two independent recording channels were 
supplied. Initially, no re-recording was in 
process, allowing both channels to be used for 





Figure 10. Phase shift correction by re-recording. 





MAGNETIC RECORDING MEDIA 


97 


recording the original transient. This aids in 
obtaining a record from at least one of the 
recording channels where the transient from 
the tape joint does not seriously interfere with 
the transient under study. Each tape loop is 
designed to accept a recording having a dura¬ 
tion of about 1/50 second, although one tape 


is used on final playback, following the second 
reversal of tape motion, to prevent it from de¬ 
tracting from the observation of the initial 
transient on the cathode-ray oscilloscope. 

The MTR is thus reasonably free of spurious 
disturbances, and provides reasonable response, 
phase, and noise characteristics to allow its 



cycle lasts for 1/25 second. The unit as finally 
constructed had a response from somewhat 
below 800 to over 30,000 c, with a signal-to- 
noise ratio of about 30 db, after re-recording. 

Figure 11 shows the operation of the system 
as used to make the initial recording. The 
transient signal is fed to an input amplifier 
and trigger circuit, which controls the opera¬ 
tion of the recording amplifier and obliterating 
head. Thus on receipt of a signal, obliteration 
stops, the signal is recorded on both channels, 
and is ready for choice of the best record, and 
re-recording. A circuit called a click remover 
[CR], somewhat similar to noise limiter cir¬ 
cuits used in communications radio receivers, 
is applied to the chosen record in such a manner 
that the loop-joint click transient is removed 
from the channel prior to re-recording on the 
now obliterated second channel. Since the posi¬ 
tion of the transient on the chosen recording 
may be determined with reference to the joint 
in both tapes, re-recording can be made with¬ 
out the second tape joint being in a position 
to interfere with the signal transient. The CR 


use for many problems. As a result of later 
work in the field of magnetic recording media 
(ER, PCT), more adequate characteristics 
could be obtained, but work under OSRD was 
discontinued before such a redesign and appli¬ 
cation were possible. 

35 MAGNETIC RECORDING MEDIA 

Military Requirements 

Initially there were no military requirements 
established for the development program on 
magnetic recording media [MRM]. Studies of 
MRM were initiated as a result of difficulties 
encountered in the development of the MTR. 
It had been hoped to eliminate joint transients 
(clicks) in the MTR tapes by plating a non¬ 
magnetic loop with a continuous magnetic 
plating. Later work on the development of 
MRM was carried out under a directive from 
Army Air Forces. 0 The main objective of this 
directive was to obtain MRM which would 

<• SC-lll 



































98 


MAGNETIC RECORDING RESEARCH 



Figure 12. Assembly of head cartridge (actual size). 


allow adequate performance in airborne ap¬ 
paratus, yet require less storage space than 
MRM available at that time. 

Ring Heads 

It was found from plated-tape studies that 
a design of special recording and reproducing 
heads, known as ring heads [RH] was neces¬ 
sary. The standard magnetic head used for 
longitudinal recording uses two pole pieces on 
opposite faces of the tape, focusing the field 
within an area limited on both ends by the 
slightly offset pole pieces. (See Figure 6.) 
These pole pieces do not produce the same 
effect with a plated tape, since between the two 


magnetic layers on the faces of the tape, a 
nonmagnetic portion is sandwiched, which en¬ 
courages undesirable leakage paths for the re¬ 
cording flux. The RH design is such as to 
concentrate the flux in the magnetic layer near¬ 
est to it, making a tape plated on one side all 
that is necessary, and eliminating the leakage 
path effects mentioned. 

Figures 12 and 13 illustrate the design and 
construction of one of the later models of this 
head. The head is essentially a ring with a 
small, carefully controlled air gap at the point 
of contact with the magnetic layer of the tape 
used as a signal carrier. A second air gap is 
provided on the opposite side of the ring. While 









MAGNETIC RECORDING MEDIA 


99 


the tape actually contacts a considerable por¬ 
tion of the ring on either side of the gap dur¬ 
ing normal operation, the effective slit width 
is always determined by the air gap itself, 
since the reluctance of the ring material is so 
much lower than that of the tape that practi¬ 
cally no flux leaks across to the tape except at 
the air gap. 

It should be emphasized that RH are inferior 
to standard heads when used with standard 
steel recording tape, probably due to the great¬ 
er thickness of the magnetic material of the 
tape which allows more spreading of the flux 
and hence less definition for high frequencies. 
However, the RH have definite advantages for 


plated or coated tapes, where the magnetic 
material is thin. All tests on ER or PCT were 
made with ring heads similar to that described. 
Some plating tests carried out on disks did not 
use RH, but used modified heads of standard 
design. 

Early Plating Developments 

While some of the earliest plating tests were 
made on a nonmagnetic metallic tape, beryllium- 
copper, these tests did not prove very satis¬ 
factory, although hope for future developments 
led to a continuation of work on tapes. To 
facilitate early plating tests and analysis of 
results, as well as to supply recording signal 



Figure 13. Assembly of recording head (actual size). 







100 


MAGNETIC RECORDING RESEARCH 


carriers for then available apparatus in use by 
the Services, it was decided to carry out tests 
on the plating of nonmagnetic disks, for com¬ 
parison with the tool-steel disks then in use. 
The disks were about 4 in. in diameter and 
3/16 in. thick, after machining and initial pol¬ 
ishing. Several different materials were used 
as a base to allow study of the effects of heat 
treatment on the magnetic plating without dis¬ 
tortion of the base itself. Prior to plating, all 
disks were carefully polished. Even after such 
polishing, it was found that variations in back¬ 
ground noise, predominantly low-frequency ef¬ 
fects, existed. This was found to depend on 
the uniformity of the plating crystallization, 
which in turn was dependent upon the uni¬ 
formity in crystalline structure of the base 
material. 

During initial tests it became apparent that 
the unrelieved strains present in a magnetic 
plating were directly related to its magnetic 
characteristics, and that heat treatment which 
tended to reduce these strains would reduce 
the quality of the plating for magnetic record¬ 
ing. 

An iron-cobalt alloy was chosen for the mag¬ 
netic film, in a ratio of 35 per cent Co to 65 
per cent Fe, after numerous trials. This com¬ 
bination appeared to have the best signal-to- 
noise ratio of the various compositions tried at 
that time. 

Studies were made of optimum plating solu¬ 
tions, current densities, pH values, plating 
temperatures, and mechanical arrangements of 
plating apparatus. The general arrangements 
are briefly noted here. 

1. The plating solution consisted of: 

360 g Ferrous chloride 

180 g Calcium chloride 

150 g Cobalt chloride 

Water to make 1 liter. 

2. The anode used was Armco iron. 

3. The current density direct current was 
about 20 amp per square foot. 

4. The pH of the solution was 1.5. 

5. The plating temperature was 70 C. 

It was found that higher current densities, 
or lower pH values, caused pitting of the plat¬ 
ing. To prevent the plating from following 
the crystalline pattern of the base material, the 


disk was rotated while being plated. A number 
of thin layers, about 4, each about 0.0002 in., 
were applied, each being polished prior to 
application of the next layer. 

A nonmagnetic base material having a fine 
grain structure was desired. Brass containing 
70 per cent copper, 29i/ 2 per cent zinc, and y 2 
per cent phosphorus, heat treated at 800 F was 
selected. 

Since the plating as deposited had substantial 
strains created in the magnetic layer, its char¬ 
acteristics appeared adequate without further 
treatment after deposition. 

A signal-to-noise ratio of 40 db, as compared 
to 20 db obtained with tool-steel disks under 
identical conditions, was obtained. An oscillo¬ 
graphic comparison of the difference in signal- 
to-noise ratio is shown in Figure 14. As a 
result of this initial success it was decided to 
expand the program on the study of MRM to 
allow a comprehensive study of various types 
and methods of plating and coating tapes, in 
the hope of obtaining materials superior, from 
the standpoint of response and signal-to-noise 
ratio, to the then available materials. 

Extended Studies 

In addition to the advantages realized in 
plated signal carriers on disks, outlined above, 
it was felt desirable to carry out extensive work 
on the development of easily produced tape or 
ribbon recording materials, along similar lines, 
even if they did not prove markedly superior 
to previously available homogeneous materials, 
such as tungsten steel tape. This decision was 
based on the fact that many of these materials 
were imported prior to the start of the war, 
and became increasingly difficult to obtain as 
the war progressed. It appeared that the dep¬ 
osition of thin magnetic layers on nonmag¬ 
netic bases might offer a more economical solu¬ 
tion, from the standpoint of availability of 
materials, and necessary manufacturing man¬ 
hours, than would the development of materials 
duplicating those previously imported. 

General Methods 

With these points in mind, several methods 
for deposition of thin magnetic layers were 
considered. They included: (1) electroplating, 
(2) evaporation, (3) suspension of fine parti- 


CONI 







MAGNETIC RECORDING MEDIA 


101 



SIGNAL ON STEEL DISK OBLITERATED STEEL DISK 


Figure 14. Signal to-noise ratios for plated disk and tool-steel disk. 


cles in a supporting medium, applied to a 
suitable base, (4) spraying, and (5) cathode 
sputtering. 

Of these methods, the first three were in¬ 
vestigated with considerable care but the two 
latter were disregarded since the other methods 
appeared to offer greater possibilities of an early 
solution. 

Previous experiments of electroplating on 
disks served as a basis for further work in 


this field. The selection of metals used in 
electroplating is limited by the plating possi¬ 
bilities of the metals either alone or in alloyed 
form. 

Evaporation is the process of condensing the 
vapor from molten metal onto a base material in 
vacuum. The selection of metals to be used 
is limited by the partial vapor pressures of the 
metals comprising the melt. 

The suspension method provides a wide 








102 


MAGNETIC RECORDING RESEARCH 


choice of magnetic materials, since the material 
is ground and suspended in a supporting medium 
before it is coated onto the base material. This 
process does not depend upon as many physical 
and chemical properties of the material as is the 
case with the other methods. 

3 - 55 Plating 

Initial plating tests were made on tapes of 
beryllium copper or manganese steel 0.002 to 
0.003 in. thick and about 0.014 in. wide. As 
stated, these tests were an extension of work 
on plated disks. Early results obtained from 
the tape-plating tests were discouraging, but 
led to the discovery of interesting data serving 
to confirm the fact that internal stresses within 
the magnetic layers were directly related to the 
performance of the magnetic medium. Signals 
recorded on these early tapes would gradually 
decay. The coercive force of the magnetic layer 
on these tapes was considerably lower than the 
coercive force of the same material when depos¬ 
ited on disks. It was suspected that the tape was 
not sufficiently rigid to withstand the consider¬ 
able forces developed by the stresses within the 
plated film, without being distorted and thus 
permitting their relief. The magnitude of these 
forces was conclusively indicated by the tape 
buckling badly when only one side was plated. 

At the time, a solution appeared difficult. 
However, further studies indicated that there 
were advantages to an extremely thin signal 
carrier from the standpoint of penetration 
effect. This change to a somewhat thinner car¬ 
rier, plus a change in plating technique, where¬ 
in a-c and d-c currents were superimposed dur¬ 
ing the plating procedure, finally allowed the 
production of a satisfactory tape. 

Since some 300 tapes and 2,000 short strips 
were plated during the course of the investi¬ 
gation, it is felt that a brief discussion of the 
main types of platings studied, some state¬ 
ments on the techniques, and an indication of 
results obtained on the best platings will suffice. 

On the basis of data available in the litera¬ 
ture and trade, the use of alternating current 
superimposed on direct current as a plating 
current seemed desirable because of better ap¬ 
pearance and adherence of the plating. In the 
course of the investigations, it was observed 


that the coercive force of almost all the ferro¬ 
magnetic platings increased considerably when 
alternating current was superimposed on the 
direct current during the plating process. It 
is significant that this effect occurs only when 
the peak value of the alternating current ex¬ 
ceeds the direct current; i.e., when the cathode 
becomes temporarily anodic during each a-c 
cycle. The effect also increases, up to a certain 
limit, with the duration of the current reversal, 
i.e., with increasing a-c to d-c ratio. Consider¬ 
ation of this and other data leads to the sug¬ 
gestion that occluded gases (particularly oxy¬ 
gen) or their metal compounds, serving as 
stress centers in a manner similar to that of 
dispersion or precipitation hardening of alloys, 
increase the trapped stresses within the plated 
material and thus increase the coercive force. 
A-c—d-c plating was used in all final plating 
tests, and the best results were obtained with it. 

Platings of the following materials were 
tested: nickel, iron, cobalt-iron, nickel-iron, 
cobalt-nickel-iron, cobalt-nickel, iron-cobalt- 
chromium, and iron-cobalt-molybdenum. 

Values of coercive force ( H c ) and remanence 
(B r ) were measured with a B-H meter de¬ 
veloped for this purpose, in which the hys¬ 
teresis curve of the material was automatically 
plotted on a cathode-ray tube, whose screen 
was calibrated in oersteds for H c and gausses 
for B r . 

Temperature, pH, current density, and a-c to 
d-c ratios were noted and controlled. Numer¬ 
ous types of plating baths, to give various crys¬ 
talline structures, and alloy compositions were 
employed. The addition of boric acid (H 3 BO: { ) 
to many of the plating solutions improved both 
the appearance and the adherence of the coat¬ 
ing. In general, especially in the case of plat¬ 
ing from Fe, Ni, and Co, the chloride solutions 
of these metals were preferred. 

A plating thickness of about 0.0003 in. was 
used in most cases, since any increase in the 
plating thickness above this value does not im¬ 
prove the values of B r or H c . As explained pre¬ 
viously, thin signal carriers offer advantages, 
at least when used with RH, from the stand¬ 
point of penetration effects at high frequencies. 

As a result of these studies, a plated tape, 
utilizing a base material of phosphor bronze, 






MAGNETIC RECORDING MEDIA 


103 



FREQUENCY IN C 


Figure 15. Frequency response of plated tape No. 59. 


was produced, with good magnetic recording 
characteristics, adequate mechanical strength 
and wear resistance, and capable of easy pro¬ 
duction. 9 The tape was identified as No. 59. 
Its characteristics are as follows: 


Response* signal-to-noise ratio See Figure 15 


Thickness 

Width 

Breaking strengtht (plated) 
Plating thickness 
Plating composition 

Coercive force, H c 
Remanence, B r 


0.002 in. 

0.014 in. 

5 lb 

0.00032 in.(one side only) 
80 per cent cobalt 
20 per cent nickel 
210 oersteds 
7,500 gausses 


* It is interesting to note that the velocity of tape No. 59 can be re¬ 
duced 30 to 40 per cent below the velocity of frequently used steel 
wire, and still produce the same response. 

t The unplated strength is about 3 lb. Thus the plating layer exhibits 
an apparent strength of about 300,000 lb per sq in., the same order of 
magnitude as found in homogeneous materials having similar mag¬ 
netic properties, produced by costly drawing, rolling, etc. 


Data on plating technique used to obtain this 
tape are of interest and illustrate a typical set 
of conditions used in these studies. 


Solution: 50 grams cobalt (as a chloride) ) per liter 
50 grams nickel (as a chloride ) l of 
25 grams boric acid \ solution 

Temperature: 70 C. 

71 H: 4.7 

A-c/d-c: 350/100 (i.e., 350 amp a-c, 60 c per sq ft. 

100 amp d-c per sq ft, currents superimposed) 

A continuous plating apparatus was con¬ 
structed to plate long lengths of tape, follow¬ 
ing continuous automatic cleaning by a solu¬ 


tion of trichlorethylene as a degreaser, followed 
by a hot solution of NaOH and NaCN. The 
plating unit is shown in Figure 16. 


Figure 16. Improved plating unit, showing filter and 
pump for dripping system. 













































































104 


MAGNETIC RECORDING RESEARCH 


Evaporation 

Equipment 

The evaporation of metal can be carried out 
in a satisfactory manner only at pressures of 
10 -4 to 10~ 5 mm of Hg. Apparatus capable of 
attaining these pressures in a reasonable time 
was constructed, with provisions for pressure 
measurement. Adequate room was provided 
within the evacuated chamber for electrical 
and mechanical facilities needed to carry out 
experiments. The chamber consisted of an 18- 
in. diameter bell jar with a steel base plate, an 
oil diffusion pump, and auxiliary mechanical 
pumps. 

Power was supplied to the evaporation 
sources through a Variac-controlled trans¬ 
former. Provisions were also made to produce 
a glow discharge in the bell jar during the 
pumping, since it has been reported that such 
a discharge cleans up the various surfaces in 
the chamber and thus aids in attaining a high- 
vacuum. P 2 0 5 driers were also incorporated 
in the system. 

Materials Studied 

While it had been hoped that some work 
could be done on the evaporation of Fe-Co-Mo 
melts, theoretical considerations of the vapor 
pressures of the various constituents made it 
appear that the evaporation of appreciable Mo 
from such a melt would be difficult, if not im¬ 
possible. Therefore it was decided to concen¬ 
trate on melts of Fe-Co, and possibly certain 
Alnico compositions. Since heat treatment of 
Fe-Co alloys cannot be expected to cause signi¬ 
ficant magnetic hardening (increase in internal 
stresses), the stresses obtained in the evapora¬ 
tion process itself were depended upon for 
these characteristics. 

Methods 

The ordinary method of evaporation of a 
metal is to place the metal on an electrically 
heated filament of refractory metal such as 
W, Mo, Ta, etc. Although these materials can 
be used at high temperatures, it was found that 
Fe-Co in solid-liquid equilibrium with these 
materials forms an alloy rich in Fe or Co, melt¬ 
ing at considerably lower temperature, about 
1500 C. Since this change will cause failure of 


a filament used in evaporation techniques, new 
techniques of evaporation from such filaments, 
and from special crucibles, had to be developed. 
It was found that filaments of W, plated with 
Fe-Co, where the mass ratio of Fe-Co to W was 
kept below the value of about 0.2, would allow 
successful evaporation from filaments, since 
the alloying effects were confined to the fila¬ 
ment surface. The thickness of layers that 
could be evaporated by this method was re¬ 
stricted to about 3 X 10 -5 in., because of the 
limited ratio of Fe-Co to W. 

To overcome this restriction, crucibles of 
BeO, formed over a heating element of W, 25 
turns of 25-mil wire, were developed. 8 These 
crucibles held a charge of 5 g, sufficient to de¬ 
posit a layer about 3 X 10~ 4 in. on a surface 15 
cm away. Evaporation rates of about % gram 
per hour were obtained at temperatures of 
1550 C. 

Actual evaporation took place onto disks 
similar to those used in the initial plating ex¬ 
periments, i.e., about 4 in. in diameter. Vari¬ 
ous base materials and surface treatments were 
used to improve the adherence and surface of 
the evaporated film. 

Measurements of the magnetic properties 
and thicknesses were made with the B-H 
meter 8 previously mentioned. 

Variables 

Most experiments were made with melts 
which produced an alloy of 35 per cent Co, 65 
per cent Fe. The following factors were studied 
for their effect on coercive force and remanence. 

1 . Source of specimen distance. 

2. Nature of backing, or base, material. 

3. Composite backings. 

4. Rate of evaporation. 

It was found that a great improvement in 
both H c and B r is obtained when the source-to- 
specimen distance is large. This is caused by 
the fact that at short distances, the backing 
material, and the evaporated film, are at high 
temperature, due to the radiant energy re¬ 
ceived from the source. At greater distances, 
lower temperatures obtain. Under these condi¬ 
tions, less annealing effect occurs, so that the 
internal stress regions are produced and main¬ 
tained more readily. This produces better val- 





MAGNETIC RECORDING MEDIA 


105 


ues of H c and B r . The adherence of the evapor¬ 
ated film to the base material is also dependent 
upon unrelieved stresses, as well as surface 
conditions, etc. Thus, while great source-to- 
specimen distances produced better films from 
a magnetic standpoint, the increased stresses 
caused peeling of the film to occur much earlier 
in the process, so that only very thin layers, 
from 1 to 10 X 10~ 5 in. thick, would adhere. 

Numerous backing materials, and various 
methods of surface finishing and cleaning were 
studied. An upper limit of 10 X 10 -6 in. for the 
evaporated films was obtained, after which 
peeling occurred regardless of the backing ma¬ 
terials or techniques used, unless the backing 
material was at a high temperature, which al¬ 
lows stress relief and produces unfavorable 
magnetic properties. 

Some composite backings were tried, using 
intermediate layers of chromium, aluminum, 
copper, and chromium-aluminum. While some 
reasonably good results were obtained with 
Cr-Al intermediate layers on a glass base, 
with the usual Fe-Co evaporation layer, the 
thickness produced, about 9 X 10 -5 in., was not 
fully adequate for magnetic recording. 

Increases in the rate of evaporation were 
found to improve the magnetic characteristics 
of the films, but reduce the adherent thickness, 
as mentioned before. Values of H c as high as 
150 oersteds, and B r of 23,000 gausses, were 
obtained. 

Conclusions 

The coercive force of the evaporated layer 
has been shown to be relatively high at low 
backing material temperatures and high evap¬ 
oration rates. Adherent thicknesses were 
shown to be low for materials with reasonable 
magnetic properties, apparently caused by as¬ 
sociated internal stresses. It is probable that 
Fe-Co evaporated films with a coercive force 
of about 80 oersteds and a thickness of 2 X 10~ 4 
in. might be produced by the use of composite 
backings. Other alloys do not appear favorable 
for use in the evaporation process. Wear resis¬ 
tance and production problems, in addition to the 
somewhat inferior magnetic results obtained, 
seem to discourage the use of this material, es¬ 
pecially in view of plated and PCT characteris¬ 
tics. 


Powder-Coated Tapes 

General 


The object of this research was to develop a 
tape consisting of a nonmagnetic backing hav¬ 
ing one surface coated with a magnetic pow¬ 
der, the performance of which would be com¬ 
parable with that of homogeneous recording 
media. A tape was developed whose perform¬ 
ance exceeds that of present available homo¬ 
geneous media. 

On the basis of cost, bulk storage, mechani¬ 
cal strength and wear characteristics, it was 
decided to concentrate this portion of the pro¬ 
gram on the development of a suitable mag¬ 
netic coating to be applied to one side of a 
cellulose acetate tape 0.25 in. wide, and about 
0.0015 in. thick. 


Early work included tests on various mag¬ 
netic powders, and methods of application to 
tapes. Among the powders tested, and some 
of the values of H r obtained with the B-H meter. 


were the following: 
Material 

Red Magnaflux powder 
Grey Magnaflux powder 
Martensitic steel No. 190 
White iron powder 
Alnico powder No. 1* 
Alnico 2A2-6a 
Alnico 2A2-7a 
Alnico 2A3-7a 
Alnico 2A3-7a 
Magnetite, Fe 3 0 4 


H c in oersteds 
50 
20 
55 
50 
120 

80 (on cellulose) 
80 (on Saran) 

84 (on Saran) 

80 (on cellulose) 
100-200 


* The various designations for the Alnico powders identify sources 
and particle sizes . 8 


Values of remanence B r were extremely low 
for the magnetite. Accordingly, the high ratio 
of coercive-force/remanence made possible the 
prediction that the material should provide 
good response characteristics as a magnetic 
recording medium. Recording tests confirmed 
this fact. 


Application to Tape 

Lacquer for application of the magnetite 
powder to the tape, primarily cellulose acetate, 
must withstand wide humidity and tempera¬ 
ture conditions, adhere to the tape under all 
these conditions, and be applicable to the tape 
without softening or weakening it. After sev¬ 
eral trials, a cellulose acetate lacquer X-6566, 
which proved entirely satisfactory, was ob¬ 
tained from the Columbus Varnish Company. 




106 


MAGNETIC RECORDING RESEARCH 


Following mixing of the lacquer and magne¬ 
tite, application may be made by several meth¬ 
ods. One method is to pass the paper or plastic 
over a roll above which is a cell with a knife- 
edge back. The cell is filled with the lacquer 
to be applied and the knife edge limits the 
coating to the desired thickness. Following 
coating, the material is passed through a dry¬ 
ing furnace in a continuous process. 

Roller coating may also be used, as may 
spraying under carefully controlled conditions. 


mile without joints or splices. Both cellulose 
acetate and paper bases were supplied. The 
final product had response characteristics as 
shown in Figure 17 which was obtained with 
an RH modified to accommodate the PCT. 
Standard parts were used, obtained from the 
head shown in Figures 12 and 13, but a suffi¬ 
cient number of laminations was stacked to 
obtain the required track width for the PCT. 
In the particular case cemented laminations 
(amounting to a stack thickness of slightly 



60 100 1000 10,000 20,000 

FREQUENCY IN C 

Figure 17. Frequency response of powder-coated tape. 


Sources of Magnetite 

Magnetite is a naturally occurring mineral, 
found in relatively high purity and large quan¬ 
tities in New York State. It is also produced 
synthetically for use as a paint pigment. Such 
magnetite has a maximum particle size of 
about 4 microns, and can be obtained at a cost 
of about 10 cents per pound. 9 It was found 
that the materials for a mile of this coated 
tape cost about 50 cents, exclusive of coating 
cost. A mile of tape will record from 40 to 90 
minutes of program with good fidelity and 
forms a reel about 16 to 18 in. in diameter. 

Conclusions — PCT 

Magnetite-coated tapes were produced by 
various contractors in lengths as long as a 


over y 8 in.) were used with flat tape guides 14 
in. wide between the side plates. 

The tape characteristics, to summarize, are 
as follows: 

Width in. 

Thickness 0.0015 in. 

H c 100-200 oersteds 

B r About 300-500 gausses 

The high ratio of H c to B, indicates a good 
response and good signal-to-noise character¬ 
istic. This is illustrated in Figure 17. 

Production cost and availability of mate¬ 
rials and producers are, and should continue to 
be, extremely favorable. 

36 CONCLUSION 

The development of new and improved mag¬ 
netic recording media, initiated as a result of 


tal 

















































CONCLUSION 


107 


work done on a magnetic transient recorder, 
has resulted in media capable of equaling or 
surpassing previously available homogeneous 
magnetic recording media, in both response and 
signal-to-noise ratio characteristics. Availa¬ 
bility of these media is excellent, based upon 
ease of production, and convenient sources of 


necessary materials. Manufacturing and mate¬ 
rial costs are low, thus making the wide use 
of such media appear desirable. 

The magnetic transient recorder develop¬ 
ment has shown one of the many possibilities 
of magnetic recording applied to measure¬ 
ment problems. 



Chapter 4 

OSCILLOGRAPHS 

By George E. Beggs, Jr. b 


INTRODUCTION 

HIS REPORT describes briefly four oscillo¬ 
graph developments under Division 17 of 
the National Defense Research Committee. Three 
of these resulted in apparatus for use by the 
Ordnance Department of the U. S. Army and 
the Bureau of Ordnance of the U. S. Navy. 
Apparatus was intended primarily for use at 
the various proving grounds, for accurate 
quantitative measurements of such things as 
blast pressures, trunnion reactions, recoil- 
cylinder pressures, barrel strains, and muzzle 
velocities. Such measurements were needed in 
connection with interior ballistic studies nec¬ 
essary for establishing gun-design principles 
and data, improvement of projectile and bomb 
shapes, the development of automatic weapons 
for aircraft, and the design of aircraft structures 
to support such weapons. 

Since measurements of this type require a 
wide range of frequencies and amplitudes to 
be covered by automatic recording apparatus, 
it was felt desirable to attempt to design‘and 
construct several different systems utilizing 
either cathode-ray tubes or mechanical oscil¬ 
lograph elements recording on photographic 
media. Accordingly, three contracts were es¬ 
tablished under the supervision of the NDRC 
at the request of the War Department. 

Project OD-73 was concerned with the de¬ 
velopment of a multi-element mechanical oscil¬ 
lograph with associated d-c amplifiers, for the 
simultaneous recording of six channels of in¬ 
formation on photographic paper. The work 
was done by the Hathaway Instrument Com¬ 
pany. 1 * 3 

Project OD-102 was concerned with the de¬ 
velopment of a three-channel cathode-ray os¬ 
cillograph with a fourth channel to introduce 
timing lines. The units were to be adaptable 
to recording on a drum camera with a fre¬ 
quency response as high as several mega- 

“ OD-73, OD-102, OD-140, AC-67. 

b Technical Aide, Section 17.1-17.2, NDRC. 


cycles, far beyond the range of the mechanical 
recording oscillograph under Project OD-73. 
The work was done at Purdue University. 4 * 7 

Project OD-140 was concerned with the de¬ 
velopment of a mobile laboratory trailer con¬ 
taining a four-channel recording cathode-ray 
oscillograph with relatively wide band re¬ 
sponse, an associated high-speed camera to 
photograph the traces, and adequate for initia¬ 
tion of auxiliary apparatus operations in the 
field (i.e., the firing of the gun), for the starting 
of the recording camera and oscillograph traces, 
and for the rapid development of the traces for 
analysis after the records are taken. The work 
was done by White Research Associates. 8 ’ 9 

From the brief description above, it can be 
seen that the three developments tend to sup¬ 
plement each other in frequency response, 
versatility, and mobility. All sets of apparatus 
were completed and delivered to the Aberdeen 
Proving Ground, Ballistics Research Labora¬ 
tory. Units developed under OD-140 for simi¬ 
lar application were in process of completion 
in September 1945 at the Dahlgren Proving 
Ground, by the Bureau of Ordnance, U. S. 
Navy. 

The fourth oscillograph development, under 
AC-67, was for the purpose of furnishing to 
the Army Air Forces Proving Ground Com¬ 
mand, Eglin Field, Florida, an instrument 
trailer, the primary function of which was to 
be the recording of time sequences originating 
from an airplane in flight and terminating on 
or near the ground. The recording was to be 
accomplished by means of recorded impact, 
sound or light impulses. The work was done 
by the Shell Oil Company. 10 

42 MULTI-ELEMENT OSCILLOGRAPH 
(OD-73) 1 * 3 

Military Requirements 

The performance requirements and general 
description of the apparatus were specified in 
some detail by the Aberdeen Proving Ground. 



108 



MULTI-ELEMENT OSCILLOGRAPH 


109 


The oscillograph is to be used to record, 
simultaneously, six phenomena of a very irreg¬ 
ular or nonrecurring type. Accurate measure¬ 
ments are to be made of both the time of occur¬ 
rence and the magnitude of the phenomena under 
investigation. The duration of the phenomena 
may be as long as iy 2 seconds. During this time, 
the sign or direction of the quantity being mea¬ 
sured may remain unchanged for periods as long 
as 1/4 second, and even then may be predominantly 
in one direction. The records may have frequency 
components of small magnitude as high as 8,000 
to 10,000 c, while those components of fre¬ 
quency as high as 5,000 c may be comparable 
in magnitude to the slower phenomena. 

Although the machine will be used as a labora¬ 
tory research instrument for a large variety of 
recording operations where facility of control is 
important, it will at times be required to produce 
a large quantity of records. It should, therefore, 
be very reliable in operation and have a simplicity 
of control that will require a minimum of atten¬ 
tion. Covers of simple design are to be placed over 
all the mechanism with the exception of those 
controls or indicators which are used during the 
routine operation of the machine. The latter 
are to be arranged for convenient manipulation 
and are to be grouped for easy explanation to 
new operators. 

The oscillograph shall include a paper-trans¬ 
porting mechanism, six recording channels 
with driver amplifiers, and apparatus for pro¬ 
ducing timing and base lines on the records. 
The paper drive shall include the motor and 
a speed-change mechanism. The six recording 
channels shall be provided with all associated 
optical equipment, shutters, the recording lamp 
or lamps, and the lamp-control apparatus. Six 
separate driver amplifiers for the recording 
galvanometers shall be installed. A timing ele¬ 
ment, separate from the six recording chan¬ 
nels, shall be provided, but the frequency-con¬ 
trolled power source for the timing device will 
not be required. The only external power re¬ 
quired, other than the source for the timing 
lines, shall be supplied by a single-phase a-c 
power line rated at 105 to 125 v, 60 c ±0.5 
per cent. The entire assembly shall be sup¬ 
ported from the floor on rubber-tired casters. 
The controls shall be at a convenient operat¬ 


ing height. The length of the assembly will not 
be restricted, but the width shall not exceed 
26 in. 

For the paper drive, a rotating drum-type 
paper holder for a record length of 65 in. and 
a width of 6 in. shall be used. A suitable cover 
shall be provided so that the machine can be 
operated in daylight, except during loading or 
unloading. The speed adjustment shall be in 
two ranges 10 to 50 and 50 to 250 in. per sec¬ 
ond. Both ranges are to be variable continu¬ 
ously or by at least eight convenient steps. 
Change in speed range may be made by chang¬ 
ing pulleys or gears, if necessary. The paper 
drive, the motor, and the speed-adjustment 
mechanism shall be completely enclosed, and 
the speed control for the high-speed range 
shall be conveniently located. An indicator, 
giving the paper speed in inches per second 
to within 8 per cent shall operate automat¬ 
ically. At any speed setting, the regulation 
as indicated by measuring the distance be¬ 
tween any ten adjacent timing lines shall be 
within 1 per cent. 

The galvanometers shall be aligned to give 
% in. between traces, with equal borders on 
each side. The width of the trace shall be not 
more than 1 y 2 times the minimum possible 
width as determined by diffraction at the gal¬ 
vanometer mirror. Base lines shall be located 
close to the second and fifth traces. They shall 
be exposed close enough to the recording point 
along the time axis so that only a negligible 
error will be caused by any probable move¬ 
ment of the film from side to side during the 
traverse between the recording point and the 
place where the base lines are exposed. The 
optical system used for exposing the base lines 
shall have a wider aperture than that of the 
recording galvanometers in order that the base 
lines can be made finer than the record traces 
without difficulties due to diffraction. Timing 
lines shall be at least % in. long, and shall 
have a width of not more than y 100 in. They 
shall be recorded perpendicular to the time 
axis by means of a galvanometer with its axis 
of rotation parallel to that of the recording 
drum. The usable timing lines shall have a 
spacing of 2 or 10 milliseconds, depending on 
whether the galvanometer is modulated with a 



110 


OSCILLOGRAPHS 


500-c or a 100-c source. Controls shall be pro¬ 
vided for adjusting the amplitude of modula¬ 
tion and the intensity of the recording light 
for optimum conditions in accordance with 
the paper speed. An adjustment shall be avail¬ 
able for positioning the timing lines anywhere 
between one edge and the center of the film. 
The accuracy of the timing lines shall be one 
part in 5,000 when the driving frequency is 
accurate to one part in 100,000. 

The recording galvanometers shall be mounted 
42 in. from the recording position on the 
paper drum. The drum and galvanometers 
shall be held in alignment by means of a 
metal bed which will allow this spacing to be 
changed to any value up to 46 in. without 
modifying the external cover over the optical 
system. The top of the bed shall be from 5 y 2 
in. to 6 in. below the optical path between the 
galvanometer mirrors and the recording point. 
The galvanometers shall be mounted, not more 
than % in. center to center, in adjustable 
holders that will accommodate either the 
Model OS3B or OS2B. The cylindrical lens 
near the paper shall be in a focusing mount 
supported without obstructing the space im¬ 
mediately below the lens. In order to increase 
the exposure, the aperture ratio of the cylin¬ 
drical lens shall be made as large as possible 
without materially affecting definition. The 
recording lamp or lamps shall have a line fila¬ 
ment and the optical distance from the fila¬ 
ment to each of the galvanometer mirrors shall 
be approximately equal. The alignment of the 
lamps and any reflecting surface in the optical 
system shall be adjustable. The recording-lamp 
current shall be adjustable for exposure con¬ 
trol. 

Rochelle-salt crystal galvanometers, Model 
OS3B or equal, shall be used. The overall fre¬ 
quency response (amplifier input to oscillo¬ 
graph trace) to sine waves shall not depart 
from a straight line by more than ±2 per cent 
from 2 to 5,000 c, by more than +2 or —10 
per cent from 5,000 to 8,000 c, by more than 
+4 per cent above 8,000 c. When a symmet¬ 
rical square wave with a frequency of 2 c is 
applied to the input terminals of the recording 
amplifiers, the wave traced on the oscillograph 
paper shall not depart from a true square wave 


by more than 4 per cent in amplitude at any 
point along the cycle. 

The galvanometers shall have a temperature- 
compensating condenser in the coupling cir¬ 
cuit. The amplifier shall be capable of supply¬ 
ing 1,000 v peak to peak to the condenser and 
galvanometer combination when a signal of 
not more than 0.2 v peak to peak is applied to 
the input terminals. In comparison to the volt¬ 
age gain at 100 v peak to peak output, the 
gain at 500 v peak to peak output shall be not 
more than 3 per cent lower and the gain at 
1,000 v peak to peak output shall be not more 
than 10 per cent lower. After a 30-minute 
warming-up period, the gain vs output voltage 
calibration shall remain constant within ±2 
per cent over a period of three hours, for line 
fluctuations of 105 to 125 v and for a line 
frequency of 60 ±*4 c. These calibrations shall 
be made with the output impedance in proper 
adjustment for attaining the specified frequency 
response and with an input impedance of 5 
megohms or more. 

The amplifiers shall have at least one in- 
verse-feedback circuit which feeds from the 
output of the amplifier back to a single-sided 
stage. The factor fij8 for this feedback circuit 
shall be at least 10. 

The overall noise, hum, vibration, or other 
short-period disturbances shall not cause the 
trace of any galvanometer to depart from a 
straight line, parallel to the base lines, by more 
than ±1 per cent of the maximum deflection 
obtained when the output of the amplifier is 
1,000 v peak to peak. The test record, in this 
case, shall be taken at a paper speed of 250 
in. per second with the amplifiers turned on 
and the input terminals short-circuited. After 
a 30-minute warming-up period and under the 
following conditions, the trace of any one gal¬ 
vanometer shall not depart from a straight line 
parallel to the base line by more than ±3 per 
cent of the maximum deflection as obtained 
when the output of the amplifier is 1,000 v peak 
to peak: (1) the input terminals of the channel 
under test shall be open-circuited; (2) the test 
record shall be run at a paper speed of 10 in. 
per second; (3) full modulation of any or all of 
the remaining galvanometers shall be allowed; 
(4) any necessary operation of the controls 


O 



MULTIELEMENT OSCILLOGRAPH 


111 


during or before recording shall be allowed. 

Drawer or cabinet space shall be provided 
for the storage of extra parts, fuses, cables, 
etc. The following parts and supplies shall be 
included with the oscillograph: 4 extra gal¬ 
vanometers, Model OS3B; 10 fuses of each 
type used; a 30-ft power cord, detachable or 
on reel; 20 extra lamps of each type burned 
at photoflood intensity; 10 extra recording 
lamps, any type; and all special tools required 
for operation or adjustment of the machine. 
All controls, lamp switches, shutter-operating 
mechanisms, etc., that must be manipulated im¬ 
mediately before or after exposure, shall be 
capable of control from external switches. A 
cam-operated switch (110-v, 5-amp rating or 
more) shall make and break a circuit without 
chatter at each revolution of the recording 
drum. The point of operation of this switch 
shall be adjustable to any position around the 
drum and the position shall be indicated to 1 
degree by a graduated circle. Near the ampli¬ 
fier panels, two blank panel spaces for stand¬ 
ard 8%xl9-in. panels shall be provided. 

Summary of Development 

A drum-type recording oscillograph utiliz¬ 
ing six direct-coupled amplifiers which drive 
an equal number of Brush-type OS3B crystal 
galvanometers through direct-coupled ampli¬ 
fiers having a frequency range from 0 to 8,000 
c, was developed and constructed. The instru¬ 
ment was built in a walnut cabinet, 73x36x26 in. 
mounted on casters for use within the laboratory. 

The amplifiers as originally designed and 
constructed had sufficient gain to give 1-in. 
peak to peak galvanometer deflection for 30- 
mv rms input. As shown in Figure 1, six am¬ 
plifiers, utilizing standard rack and panel 
construction, are provided on either side of 
the main cabinet, plus a seventh spare ampli¬ 
fier and two power-supply chassis in the center 
portion of the cabinet. The complete assembly 
is controlled from a sloping control panel. The 
rear portion of the cabinet contains the optical 
system, the recording drum, the variable-speed 
drive motor and controls, and the thermostatic 
galvanometer chamber. 

Records are made on strips of sensitized 
paper on film 6 in. wide at chart speeds be¬ 


tween 10 in. and 250 in. per second. Timing 
lines, spaced 1/100 second apart, are included 
on the chart, and a stroboscopic system afford¬ 
ing timing lines of any frequency up to sev¬ 
eral thousand cycles per second is provided 
to facilitate analysis of the galvanometer 
traces for amplitude and frequency. 

Six high-impedance input channels are pro¬ 
vided for the introduction of signals from 
piezoelectric devices or other similar circuits 
utilized in the measurements for which the in¬ 
strument was designed. 

42 3 Description and Technical Information 

The various available galvanometers were 
investigated to determine which could suffice 
for the frequency response and linearity re¬ 
quirements. The Brush-type OS3B crystal gal- 



Figure 1 . Hathaway type Sll-A 6-element oscillograph. 

vanometer was finally decided upon as satis¬ 
factory. However, it required accurate tem¬ 
perature control of the crystal chamber to pre¬ 
vent variations in both frequency and ampli¬ 
tude response. This particular galvanometer 
was undesirable because it has a very high 
resonant peak at a frequency of 11,000 c, just 
outside the band of frequencies requested in 
the specifications. Furthermore, it is extremely 
sensitive to voltage overload which would dam¬ 
age the crystal. Thus, in the amplifiers and 
associated recording equipment, it was neces¬ 
sary to incorporate appropriate compensation 
and protective circuits, to prevent the resonant 
peak from influencing the frequency-response 
characteristics (including transient response) 
and to preclude damage to the crystal element 
regardless of the input voltage to the record¬ 
ing unit. 










112 


OSCILLOGRAPHS 


It became necessary to select OS3B galvanom¬ 
eters from a number of units obtained from 
the Brush Development Company, in order to 
obtain six sufficiently “identical” in character¬ 
istics to make possible the use of a semi-stand¬ 
ard amplifier for each channel. Some variation 
was provided in the amplifier circuits to take 
care of a few of the variations remaining in 
these selected units. 

The amplifier requirements originally con¬ 
templated made it appear possible to use either 
a d-c amplifier or a very low-frequency re¬ 
sponse a-c amplifier. Since a-c amplifiers with 
good low-frequency response usually have long 
time constants, they tend to exhibit blocking 
characteristics under overload conditions. This 
makes it difficult in the presence of momentary 
overloads to produce records wherein anything 
save the overload phenomena is faithfully re¬ 
corded. Accordingly, it was decided to use a 
modified form of a d-c amplifier originally de¬ 
scribed in technical literature in 1941. 11 Some 
of the modifications included the addition of an 
output stage comprised of a pair of 12J5 tubes 
in push-pull to give adequate driving voltage 
for the galvanometer elements. Fortunately, 
the gain requirements were not excessive, since 
an input of 30 mv was required to provide an 
output of approximately 330 v, resulting in a 
desired gain of approximately 10,000, or 80 db. 
Frequency-response characteristics of ±2 per 
cent from 1 c to 5,000 c, and not over 4 per 
cent rise or 10 per cent drop between 5,000 
c and 8,000 c, necessitated considerable modi¬ 
fication of the amplifier circuits in conjunction 
with the galvanometers as previously men¬ 
tioned. Two frequency-compensating networks 
were added to the amplifier, one of these being 
a high-frequency filter with rather sharp cut¬ 
off characteristics placed in the plate circuits 
of the output tubes. This circuit prevented the 
frequencies in the vicinity of 11 kc and above 
from being applied to the galvanometer, thus 
avoiding excitation of the galvanometer at its 
resonant frequency. In view of the variation 
between galvanometers, it was necessary to 
make this circuit variable to operate correctly 
with individual units. 

There was also included an inverse-feedback 
circuit frequency selective in itself to some de¬ 


gree, to allow the mid-frequency range to be 
adjusted to lie within the specifications. 

The high input impedance required of the 
amplifier circuit made the use of triodes in the 
input stages somewhat difficult, but in view of 
the regulated voltages necessary, it was de¬ 
cided to continue the use of triodes with their 
associated cathode-compensation circuits for 
drift elimination and to add an input network 
to maintain the input impedance at a value of 
1 megohm or greater, from 0 to 8,000 c. The 
amplifiers finally incorporated as part of the 
equipment met the majority of the require¬ 
ments. 

The main optical housing of the oscillograph 
contained in the rear portion of the case con¬ 
sisted of the six galvanometers, a plano-convex 
lens of 42-in. focal length mounted as an in¬ 
tegral part of each galvanometer, a recording 
lens to focus the trace on the paper or film, a 
shutter and a recording lamp, two optical base¬ 
line markers, and a timing-line system consist¬ 
ing of a neon lamp or synchronous motor-driven 
shutter. The neon lamp can be supplied with 
base frequencies from external timing systems. 
The synchronous motor-driven shutter provides 
fine-line traces on the record every 0.01 sec¬ 
ond and double- and quadruple-width lines 
every 0.05 and 0.1 second, respectively. In ad¬ 
dition to these two timing devices, a bifilar 
galvanometer with an 18-in. focal length lens 
was supplied, to be used if desired. 

To maintain uniform galvanometer charac¬ 
teristics, the chamber includes a thermostat¬ 
ically controlled switch and heating and cooling 
system utilizing dry ice and a blower to main¬ 
tain the temperature at 85 F, regardless of 
ambient temperature conditions. The switch has 
two sets of contacts which energize either the 
heater or the cooling blower to maintain this 
temperature control. 

The actual recording drum and drive system 
is also located within the back part of the main 
cabinet. The drive consists of a motor whose 
speed may be varied by changing controls in 
the thyratron circuit. Speed control of the sys¬ 
tem is relatively good over a reasonable range. 
In this particular system step pulleys are used 
to avoid too wide a range of motor operation 
for the range of drum speeds needed, which 





CATHODE-RAY OSCILLOGRAPH 


113 


must provide a maximum paper or film speed 
of 250 in. per second. Since the entire oscil¬ 
lograph is mobile, it can be wheeled into a dark 
room where the drum may be loaded under 
safe light conditions; or with the addition of 
a lightproof portable canopy, the drum may be 
loaded in daylight. Paper is held on the periph¬ 
ery of the drum by jam-nut and take-up knob 
arrangement. 

The basic amplifier and power-supply cir¬ 
cuits are shown in Figure 2. 


3. Beam-triggering circuits to provide for 
turning three cathode-ray beams on and off at 
the beginning and end of a record. 

4. Time-marking and trigger circuits for plac¬ 
ing timing marks on drum-mounted film. 

5. Power supplies for three cathode-ray 
tubes to furnish the following voltages: 

a. An accelerating voltage adjustable from 
4,000 to 15,000 v negative with respect to 
ground. 

b. A positive voltage for use with intensifier- 



43 CATHODE-RAY OSCILLOGRAPH 
(OD-102) 4 - 7 

4-31 Military Requirements 

The circuit components required for the high¬ 
speed cathode-ray drum-camera oscillograph are 
the following: 

1. Three d-c and three a-c amplifier units 
with power supplies. 

2. Beam-deflecting circuits for controlling 
and shifting beams of three cathode-ray tubes 
in order to obtain a long record on a revolving 
drum. 


type cathode-ray tubes, voltage to be vari¬ 
able from 4,000 to 15,000 v. 

c. Filament supplies having center-tapped 
transformers with voltage selecting or ad¬ 
justing device to accommodate the dif¬ 
ferent filament voltages used in cathode- 
ray tubes. Three separate transformers 
and control devices are required. 

6. Three amplifiers for modulating the cath¬ 
ode-ray beams with either positive or negative 
impulses to place timing marks on cathode-ray 
beams. 

7. Three sweep circuits and amplifiers. 
























































































































114 


OSCILLOGRAPHS 


The performance requirements and general 
description of the apparatus were specified by 
the Aberdeen Proving Ground, as follows. 

The drum camera shall consist of a drum, 
7 in. wide, mounted on a vertical shaft and 
enclosed in a light-tight housing which can be 
sealed air tight and provision made for exhaust¬ 
ing the air pressure to a few millimeters of 
mercury. An arrangement to reduce the air 
pressure shall be incorporated in order to de¬ 
crease windage resistance and film flutter. At 
three positions, separated by 60 degrees, shall 
be placed high-speed lenses which are corrected 
to compensate for the curvature of the screen 
of the cathode-ray tube. At a fourth position 
provision shall be made for permanently 
mounting an optical system and apparatus for 
placing timing marks on the film which is 
mounted on the drum. The three oscillograph 
lenses shall be of special design and have air¬ 
tight seals at the end next to the camera. Syl¬ 
phon bellows shall be used to provide for seal¬ 
ing the lenses to the air-tight drum and at the 
same time allow for focusing the lenses. The 
mounts for the lenses are to be brackets which 
are an integral part of the camera housing and 
table. Rack-and-pinion type focusing mecha¬ 
nisms shall be mounted on the bracket and carry 
the lenses. The drum shall be driven by a 10-hp 
d-c motor whose speed is controlled by a Ward 
Leonard motor-generator type of speed con¬ 
trol. Provision is made for loading a strip of 
film or paper onto the drum through a door 
which is then made air tight by means of 
double gasket seals. 

Three rubber-tired dollies are to be provided 
for mounting the cathode-ray tubes, amplifiers, 
and power supplies. Detachable coupling links 
provide for lifting two end wheels of the dolly 
clear of the floor and rigidly anchoring the 
dolly to the camera table. The dollies are to be 
constructed in the form of tables and have 
provision for mounting standard radio-type 
panels on the sides and back. The cathode-ray 
tube holder is to be mounted on the table top. 
At the back of the table top a panel rack is to 
be supplied to provide for mounting the deflec¬ 
tion amplifier immediately over the socket end 
of the cathode-ray tube. Short direct leads to 
the deflection plates will thus be possible. Each 


dolly and its associated equipment will con¬ 
stitute a complete and independent cathode-ray 
oscilloscope and should be provided with de¬ 
flection amplifiers, power supplies, sweep cir¬ 
cuit, cathode-ray power supplies and control 
circuits, beam-modulating circuit, single-trace 
sweep, synchronizing circuits, circuits for trig¬ 
gering the beam on and off, and means for 
coupling to the amplifiers and disconnecting 
the amplifier outputs from the oscillograph tube 
when it is desired to use the amplifier for other 
purposes. 

Since types of cathode-ray tubes change 
rapidly, provision shall be made for using dif¬ 
ferent types. Only tubes having accelerating- 
voltage ratings in excess of 5,000 v will be 
utilized. The rectifier or rectifiers supplying 
the accelerating voltage will be grounded at 
the positive end. The accelerating voltage shall 
be manually and continuously adjustable from 
4,000 to 15,000 v. The focusing voltage and 
accelerating voltage shall be separately vari¬ 
able, but a single control shall permit adjust¬ 
ment from 4,000 to 15,000 v without changing 
the ratio or relative value of the two voltages. 
This will provide for focusing the tube at low 
voltage and then increasing the voltage until 
the desired writing speed is attained without 
the necessity of focusing at the high voltage. 
Controls shall be provided for beam intensity, 
screen-grid or first-accelerating voltage, focus¬ 
ing voltage, second-anode or accelerating volt¬ 
age, beam “on-off,” horizontal and vertical 
positioning, fine and coarse adjustment of 
sweep frequency, synchronizing, z axis, sweep 
amplitude, test signal, vertical and horizontal 
amplifier input attenuators, single sweep, trig¬ 
ger beam “on and off” with variable time delay 
and variable time on. In addition to the accel¬ 
erating voltage mentioned above (4,000 to 
15,000 v, positive grounded), a voltage posi¬ 
tive with respect to ground shall be provided 
for use with tubes having intensifies, or ac¬ 
celeration of the beam after deflection. This 
voltage shall be variable from +4,000 to 
+15,000 v. 

Two vertical amplifiers should be provided 
for each dolly. These amplifiers should be es¬ 
sentially wide-band alternating current or 
direct-current and have a maximum gain of 


rnjirifMiiirtTTftT 



CATHODE-RAY OSCILLOGRAPH 


115 


60,000 to 100,000 times. The amplifiers should 
have sufficient output-signal capacity to pro¬ 
duce a 5-in. pattern with less than 5 per cent 
amplitude distortion when 5,000-v accelerating 
potential between cathode and anode No. 2, 
and an intensifying voltage of approximately 
15,000 v between anode No. 2 and the inten- 
sifier electrode are used on the cathode-ray 
tube. The frequency response should be essen¬ 
tially flat from zero to 1 me. This should be 
accomplished by two amplifiers, a d-c and an 
a-c coupled type. The d-c unit should be flat 
to 100 to 200 kc. An a-c coupled amplifier hav¬ 
ing a flat frequency response from approxi¬ 
mately 5 c to 4 or 5 me should be supplied to 
provide for the high-frequency response. The 
above signal-amplitude and frequency-response 
requirements imply that the amplifiers should be 
capable of producing a still larger signal volt¬ 
age but with more distortion and that they 
will amplify, with less gain, frequencies above 
those specified. Attenuators should be provided 
with step adjustments to permit selection of a 
small part of the input signal. The input imped¬ 
ance of the amplifiers should be adjustable, or 
separate impedance pads should be provided to 
permit operation in approximately 1,000-ohm, 
0.1-, 1-, and 10-megohm ranges. The amplifier 
would not be expected to meet the high-fre¬ 
quency requirements stated above when the 
highest impedance input circuit is used. 

The horizontal amplifier should have ap¬ 
proximately the same characteristics as the 
vertical, except that the gain need not be more 
than 2,000 times. Input and output connections 
should be provided to permit the use of the 
amplifier with external sweep voltage or for 
other applications. Switching arrangements 
should provide for selecting the output of the 
sweep generator. 

All power supplies should operate from a 60-c, 
115-v, single-phase line. The power supply for 
the vertical amplifier consists of at least two 
separate units. A supply having extremely good 
voltage regulation by vacuum tube and low 
effective impedance should be used for the low- 
voltage stages. A separate high-voltage supply 
designed to deliver 1,500 to 1,800 v should be 
used for the plate supply of the power tubes 
which drive the deflecting plates. Decoupling or 


padding circuits should be used sufficiently often 
that regeneration will not occur due to the 
power-supply impedance. A similar power sup¬ 
ply should likewise be used for the horizontal 
amplifiers. 

The sweep-signal generator should produce 
the conventional type of sawtooth wave form. 
The voltage-time relations should be as nearly 
linear as it is practical to attain. The usual con¬ 
trols should be provided, i.e., fine and coarse fre¬ 
quency control, amplitude, synchronization to 
internal, external, and line frequency. The sweep 
frequency should be variable from a few cycles 
to 100,000 c. 

The ^-axis amplifier should be of the conven¬ 
tional type except that its coupling circuits 
should be designed to have sufficient voltage in¬ 
sulation to withstand the accelerating voltage. 
It should be capable of intensifying or suppres¬ 
sing the beam. 

No provision has been made for shifting the 
large drum along its axis, due to the difficulties 
involved in such an operation. Since the record 
trace must not move over the same portion 
of the film for more than one revolution of the 
drum, it is necessary to provide a circuit for 
shifting the base line or zero-signal-level line 
as the record is made. This may be done in 
one of two ways. A steadily changing voltage 
may be applied to the deflecting plates or a 
step voltage could be applied instead. If a volt¬ 
age having a constant rate of change can be 
obtained, it would be preferred, since it would 
produce a spiral on the drum and result in a 
continuous record. It is further desirable that 
the rate of change of the voltage be variable 
in order to control the spacing between the 
successive traces on the drum for a given drum 
speed. This would provide for adjusting the 
length of record or the length of time that is 
recorded, thus adjusting the time scale to cor¬ 
respond to the phenomenon being investigated. 

Since it is difficult to adjust by manual or ex¬ 
ternal automatic means the time delay between 
the opening of the camera and the occurrence 
of the signal to be recorded, it is necessary to 
build into the equipment a circuit for turning 
the beam on and off. This circuit should have 
the time on adjustable from 0.01 to 1 second. 
It should act either through the beam-intensity 






] 16 


OSCILLOGRAPHS 


control or through one of the amplifiers to de¬ 
flect the beam off the field of the cathode-ray 
tube screens when the camera is in the stand¬ 
by position. 

A time-delay circuit should be provided to 
control the interval between the turning on of 
the beams and the occurrence of the phenome¬ 
non to be recorded. It should be possible by 
automatic means to turn on the beam either be¬ 
fore or after a signal is made available for con¬ 
trol of the phenomenon under test. For exam¬ 
ple, if it is desired to investigate a voltage tran¬ 
sient which occurs almost instantly after the 
trigger signal becomes available, the camera 
should be turned on before the signal is applied 
to the circuit under investigation. If, on the 
other hand, it is desired to measure the velocity 
of a projectile as it passes through two screens 
placed 100 ft in front of the gun, and the gun 
is to be fired by a solenoid-operated trigger, 
the camera should be turned on after approxi¬ 
mately the time required for the trigger motor 
to fire the gun plus the hang-fire and travel 
time to a position near the first coil. A time 
range of ±0.1 second, adjustable in approxi¬ 
mately 0.001-second intervals, should be ad¬ 
equate. 

A circuit for placing time marks on the film 
at a rate of 1,000 or 10,000 per second is re¬ 
quired. Since the timing marks should not 
overlap on successive revolutions of the drum, 
it appears that a cathode-ray tube with as¬ 
sociated circuits and optical system should be 
used. Frequencies of 1,000 or 10,000 c should be 
available from a quartz-controlled frequency 
standard. It is desirable to have the marks 
in the form of short narrow vertical lines or 
well-defined dots. Electric means can be used 
to convert the wave form of the time signal 
into either dots or lines. By intensifying the 
beam and deflecting it simultaneously, short 
lines may be obtained. The same spiraling volt¬ 
age that is applied to the signal-amplifier chan¬ 
nels may be used to spiral the timing lines. 
The application of the timing lines should be 
synchronized with the on-off action of the trig¬ 
ger circuit. 

Standard rack and panel with sub-base con¬ 
struction should be used wherever possible. 
Unit type of mounting should be used in order 


to provide for flexibility in choosing the type 
of functions desired and for ease in servicing 
and replacement. 

4-3-2 Summary of Development 

Three complete recording oscillographs, in¬ 
cluding a-c and d-c amplifiers, regulated power 
supplies, input attenuators and sweep circuits, 
were constructed on dollies (designed and built 
by another contractor) as the major units of 
the cathode-ray drum-camera oscillograph. Pic¬ 
tures of these dollies are shown in Figures 3 
and 4. These three signal units in combination 
with a fourth timing unit, mounted in a sep¬ 
arate rack, comprise the complete oscillograph 


■I 



Figure 3. Oscillograph operating panel. 








CATHODE-RAY OSCILLOGRAPH 


117 



assembly utilized in conjunction with a drum 
camera constructed for the War Department 
under a different contract. The timing unit 
supplies various timing and control pulses for 
the three signal channels to allow the introduc¬ 
tion of appropriate timing pips and spiraling 
voltages for vertical sweep, etc. The timing- 
unit cabinet rack is shown in Figure 5. All of 
the various circuits and units are coordinated 
by a master-control panel which serves to con¬ 
trol all beams simultaneously to allow appro¬ 
priate interrelation between the beams when 
recorded on the high-speed drum camera. In 
actual operation, the three signal-channel os¬ 
cillograph tubes and the timing-signal tube are 
grouped at 60-degree intervals around the drum, 
the tube faces being imaged on the recording 
drum by lenses mounted as an integral part 


of it. The dollies are thus backed up in posi¬ 
tion such that the tubes face the recording 
drum and the separate control panels face out¬ 
ward, while the master-control panel is placed 
at any convenient location near the recording 
drum to allow control of both the photographic 
and electronic operations. The cathode-ray 
tubes used are 9 in. in diameter, and utilize a 
short persistence screen (P-5) with a total ac¬ 
celerating voltage of 9,000 to give adequate 
writing speed for high-frequency-response re¬ 
cording. 

Two amplifiers are available for use, one at 
a time, with any signal channel. When the first 
of these is used, an overall gain of 3,500 and 
a frequency response of 5 c to 1 me are realized. 
For the second, the frequency response is from 
direct current to slightly less than 1 me, with 
a maximum gain of 5,300. 

For horizontal deflection, there is the usual 
sawtooth time-axis generator having a fre¬ 
quency range from 8 to 120,000 c, accomplished 
by the use of high-vacuum tubes. 


Figure 5. Timing-unit cabinet rack. 


Figure 4. Front view of oscillograph. 



















118 


OSCILLOGRAPHS 


A single-sweep time axis is also provided to 
allow analysis of transients under various op¬ 
erating conditions. This single-sweep circuit 
has a range of 5 microseconds to 1.5 seconds. 

The timing-pulse generator, previously men¬ 
tioned, provides iy 2 -microsecond timing mark¬ 
ers to be inserted into oscillograms at a rate of 
10,000 or 1,000 per second, as desired. Finally 
a z-axis amplifier operates on the cathode-ray 
beam to intensify the timing pulses or the en¬ 
tire sweep oscillogram, or, if used with reverse 
polarity, to allow blanking of the return or 
other undesirable portions of the trace. 


pulses to initiate the desired phenomena prior 
to initiation of single sweep or spiraling sweep 
for the records to allow appropriate portions to 
be recorded on the film at the correct times. 
When the complete apparatus has been adjusted, 
one single operation automatically begins the 
complete measurement. 

4 3 3 Description and Technical Information 
The a-c and d-c vertical amplifiers, the vari¬ 
ous sweep systems, timing-mark devices, input 
attenuators and impedance-matching units, 
phase inverters and signal-mixing circuits, 
master-control and calibration circuits, plus 



Figure 6. Block diagram of oscillograph unit. 


The master-control unit, in addition to gov¬ 
erning the various sweep voltages which may 
be applied simultaneously to all three signal 
channels, also generates a spiraling voltage, 
i.e., a single vertical-sweep voltage which is ap¬ 
plied to all oscillographs equally and simultane¬ 
ously. Thus, when the oscillograph channels are 
used with the rotating drum, the drum rota¬ 
tion provides the horizontal time axis, and 
intelligence to be recorded is superimposed on 
the spiraling voltage. This allows intelligence 
signals to be recorded for several revolutions 
of the drum without overlap. 

The master-control unit performs numerous 
other functions, such as furnishing control 


associated power supplies, cathode-ray tubes, 
and optical systems, utilized with the equip¬ 
ment, comprise a rather complex interlocking 
circuit array. It is difficult to simplify descrip¬ 
tion of a system of this type, since oversimpli¬ 
fication leads to a misunderstanding of the 
capabilities of the apparatus, while adequate 
explanation entails voluminous description and 
circuit detail. 

The basic system is shown in Figure 6, a 
block diagram of the oscillograph, broken down 
to show one main oscillograph circuit. This 
particular block diagram is repeated in tripli¬ 
cate for the three signal channels and supple¬ 
mented by a timing unit noted in Figure 7. 
























































MOBILE MULTI-CHANNEL CATHODE-RAY OSCILLOGRAPH 


119 


To develop adequate driving voltage to deflect 
fully the cathode-ray beam of the 9-in. cathode- 
ray tubes operated with accelerating poten¬ 
tials as high as 10,000 v, output tubes capable 



Figure 7. Block diagram of oscillograph timing unit. 


of producing these voltages are needed. Con¬ 
sideration of this problem led to the employ¬ 
ment of Hytron-type HK-257B tubes in the 
output stages, since they were capable of sup¬ 
plying the necessary deflection voltage and had 
sufficiently low input capacity to make possible 
the wide frequency response desired. The over¬ 
all frequency-response characteristics of the 
a-c and d-c amplifiers are noted in Figure 8. 

A block diagram of the a-c and d-c amplifier 
circuits and attenuators can be expanded brief¬ 
ly by reference to Figures 9 and 10. These draw¬ 
ings show the a-c and d-c input attenuators, 
the associated amplifiers and output circuits, 
and points of introduction for timing pulses, 
synchronizing signals, sawtooth sweep and 
spiraling sweep. Study of these circuits re¬ 
veals that cathode followers are used in numer¬ 
ous instances as impedance transformers and 
coupling devices having wide-band frequency- 
response characteristics to d-c as well as a-c 
signals. The input attenuators are also com¬ 
pensated for linear frequency-response char¬ 
acteristics. In addition to the circuits shown, 
a separate cathode follower is supplied as an in¬ 
put impedance adaptor, to allow input impe¬ 
dance of the order of 100 megohms to be avail¬ 
able for use with certain input devices, such 
as piezoelectric strain gauges. 


The complexity of the construction and de¬ 
velopment makes it undesirable to go into fur¬ 
ther detail in this report. The final report 0 and 
an instruction manual 7 issued under the con¬ 
tract give adequate details on the circuit opera¬ 
tion and performance characteristics. To illus¬ 
trate types of records obtainable with this 
equipment, a sine wave trace with 10-kc timing 
pulses introduced from the timing generator 
unit into the amplifier is shown in Figure 11. 

It should be emphasized that the apparatus 
developed for the NDRC under this particular 
directive does not comprise a complete record¬ 
ing cathode-ray oscillograph; but, in conjunc¬ 
tion with a drum camera and laboratory dollies 
supplied by the Aberdeen Proving Ground con¬ 
tracts, a complete recording oscillograph was 
derived. 

It should be noted that the separate oscil¬ 
lograph dollies can be removed from connec¬ 
tion with the recording drum, described above, 
and used as entirely independent oscillographs 
for nonrecording observation of various cir- 



.001 .01 0.1 1.0 10 100 1000 10,000 
FREQUENCY IN KC 

Figure 8. Amplifier characteristics. 


cuits, since each oscillograph contains its own 
horizontal and vertical amplifiers and horizon¬ 
tal sweep circuits, attenuators, positioning con¬ 
trols, etc. 

4-4 MOBILE MULTI-CHANNEL CATHODE- 
RAY OSCILLOGRAPH (OD-140) 8 ’ 9 

441 Military Requirements 

The final specifications for this development 
resulted from conference between the con¬ 
tractor and Army and Navy representatives, 
and from development work and investigations 
by the contractor. They are as follows: 

This oscillograph is intended for measure- 











































120 


OSCILLOGRAPHS 




bt 







Ci 

W 



































































































































































































































































MOBILE MULTI-CHANNEL CATHODE-RAY OSCILLOGRAPH 


121 



ment of blast pressures, strains, bore pres¬ 
sures, travel time, velocity of the projectile in 
the barrel, muzzle velocity, and other transient 
phenomena associated with ordnance engineer¬ 
ing. The oscillograph is to have the highest 
fidelity of response to both high and low fre¬ 
quencies and the greatest electric and mechani¬ 
cal stability consistent with facility of opera¬ 
tion and ease of interpretation of the records. 

More specifically, the unit is to have four 
channels, each consisting of input terminals, an 
amplifier, and a cathode-ray tube. The four 
cathode-ray tubes are to be mounted on a single 
panel with their centers at the corners of a 
60-degree equilateral parallelogram. The four 
tubes are to be photographed by a single lens 
on a strip of moving photographic paper. The 
horizontal plates of each tube are to be con¬ 
nected to the output of a high-quality ampli¬ 
fier. Timing marks are recorded on the paper 


simultaneously with recording, to provide time- 
axis calibration. Voltage calibration circuits are 
to be incorporated as well as some of the various 
auxiliary circuits described below, including 
sequence timers, sweep circuits, square-wave 
generators, etc. 



Figure 11 . Sine wave trace with 10-kc timing pulses. 





























































122 


OSCILLOGRAPHS 


The mobile models of this equipment will be 
built into Army K-72 trailers complete with 
all auxiliary equipment, including a dark room 
and workshop. An auxiliary truck will be pro¬ 
vided for limited moving of the trailer unit 
and primarily for carrying the motor-genera¬ 
tor equipment. The laboratory models will be 
semiportable and adapted to semipermanent 
indoor installation. All units will be complete 
and self-contained and ready to operate ex¬ 
clusive of the actual measuring elements such 
as piezoelectric pickups, strain gauges, etc. 

Each amplifier should be capable of deflect¬ 
ing the spot over the full useful range of the 
screen with 0.1 v input. The frequency response 
should be flat from direct current through 50 
kc, and be down not more than 3 per cent at 
100 kc. The amplifiers should have a linear 
phase-shift characteristic over the useful range 
of frequencies. They should produce a deflec¬ 
tion of the spot as nearly linearly proportional 
to the input as is possible over the useful range 
of the screen. They should have a high degree 
of stability to variations of temperature and 
voltage and be as nonmicrophonic as is reason¬ 
ably possible. It is very desirable to provide 
anti-shock mounting and acoustic insulation. 
The amplifiers should operate from power sup¬ 
plies fed from 110- to 115-v 60-c single-phase 
mains, or from the gasoline-driven motor-gen¬ 
erator sets which are to be supplied. The gain 
measured with maximum deflection of the spot 
must be constant as specified over the full fre¬ 
quency range. Both high-impedance (10 
megohms) and low-impedance (1,000 ohms) 
inputs either single-ended or balanced are to 
be provided. The output stage is to be push- 
pull. Means should be included to show the 
operating point of each stage (e.g., a panel 
galvanometer with a switching arrangement so 
that each stage can be balanced separately). 
Gain control is to be provided by the use of 
an input attenuator calibrated in convenient 
steps. The spot-centering control may be the 
last balance control. External connections 
should be supplied for the input and output of 
the amplifiers. A suggested method is the pro¬ 
vision of a patch-cord panel for connection be¬ 
tween the usual signal input source from the 
pickups or external signal source and the input 


of the amplifiers. This would provide a con¬ 
venient method of choosing between balanced 
or unbalanced, high- or low-impedance inputs 
of the amplifier and would also allow choice of 
the amplifier to be used on a particular channel. 

Provision should be made for sharp time 
marks on the paper derived from a crystal 
standard. If reliable flashing glow tubes are 
available, a suggested method is to place such 
tubes on the oscilloscope panel on a horizontal 
axis between the upper and lower sets of cath¬ 
ode-ray tubes. There would be three tubes on 
one side and one tube on the opposite side. The 
film record would thus have a series of single 
dots along one side and single, double, and 
triple dots on the opposite one. The single dots 
would correspond to 1-millisecond intervals, 
the double dots to 5-millisecond intervals, and 
the triple dots on one side to either 10- or 100- 
millisecond intervals. The choice between 10 
milliseconds or 100 milliseconds would be made 
by means of a suitable switch. Regardless of 
the appearance of the timing marks (lines or 
dots) it is essential that they be at least as 
sharp as the recorded traces of the cathode-ray 
tubes. A crystal standard timing source is pre¬ 
ferred such as the Gibbs oscillator or similar 
type developed by the contractor. 

A camera using strip-paper film 70 mm wide 
is desired. The paper should be delivered from 
the camera either directly into the dark room 
or into a tight box which may be carried into 
the dark room. The Smith type of tank develop¬ 
ing is preferred. The mechanical design of the 
camera should be such that the paper can be 
accelerated to full speed within 25 milliseconds 
and that film spoilage or breakage be mini¬ 
mized. Provision should be made for a storage 
roll of at least 200 ft of paper film. The camera 
should be completely automatic in operation 
as long as the film supply lasts and there should 
be provision for preselecting the paper speed 
and any length of record from 1 ft to 20 ft. 
There should be an indicating counter to reveal 
the amount of footage remaining on the storage 
roll and an automatic device to cut the film at 
the right length at the end of a record run. 
There should be no necessity for rethreading 
film between records until the storage roll has 
to be replaced. There should also be provision 





MOBILE MULTI-CHANNEL CATHODE-RAY OSCILLOGRAPH 


123 


for indicating on the film the record number 
and the time, just before a record is taken. It 
is preferred that this operation be automatic 
and take place just before voltage calibration. 
Either step-wise or continuous control of paper 
speed would be acceptable. It is desired that 
this speed be variable from 10 in. per second to 
at least 250 and preferably to 500. It is sug¬ 
gested that the cathode-ray tubes be photo¬ 
graphed at an 8 to 1 reduction. The lens should 
have a focal length of 6 to 6.5 in. and an aper¬ 
ture of F/2.5, or larger. The image should be 
sharp over the entire field of view. There should 
be a control to intensify the beam to photo¬ 
graphic brilliance automatically just before the 
camera is started. The camera itself should be 
operated by an electromagnetic control so that 
it can be started by the sequence timer, push 
button, or other switching means either within 
the trailer or at some remote point. Means 
should be provided for properly focusing the 
camera and for easily checking its focusing 
at any time. 

The use of the 9-in cathode-ray tube, DuMont 
EX-1829, was decided upon. Originally, a 2514C9 
tube was considered, but tests made by the con¬ 
tractor proved that the former had more ad¬ 
vantages. While the electric characteristics and 
screen brightness are the same for both tubes, 
the DuMont tube has a flat face and is more 
stable and easier to produce. Provision should 
be made for adjusting the inclination of the 
tube for the best compromise between linearity 
of deflection and focus. 

It is desired that voltage calibration in ten 
steps be recorded on film before each run. It 
has been suggested that this calibration be ap¬ 
plied while the camera is coming up to speed 
or that the calibration be photographed before 
the film is accelerated, and possibly simultane¬ 
ously with photographing the time and identi¬ 
fication number. In the latter case the calibrat¬ 
ing switch could be either manually operated 
or motor driven. The final decision must await 
tests on the present camera, plus further con¬ 
sultation with Aberdeen and Dahlgren Proving 
Grounds. In any case the switching device must 
provide positive contact without chatter, and 
the time constant of the circuits involved should 
be less than one-tenth the minimum duration 
of the step. 


A device should be provided for voltage, 
charge or resistance calibration, depending 
upon whether the equipment is being used 
for recording circuit transients, piezoelectric 
gauges, or strain gauges, respectively. The 
voltage calibration should be against a stand¬ 
ard cell. The range of calibration steps should 
be from 0 to full-scale in ten equal steps. The 
amount of voltage should therefore be adjust¬ 
able to accord with the gain setting of the 
amplifier. For charge calibration there should 
be provided a decade capacitor so connected 
that it can be charged from a constant-voltage 
source and discharged into the ballast capac¬ 
itance. A mechanical arrangement should be 
provided so that this operation can be repeated 
ten times in succession. The calibrating capa¬ 
citor should remain connected into the ballast 
capacitance while the test is being recorded. 
For resistance calibration a rotating contactor 
similar to that used for voltage calibration 
could be used. Means should also be provided 
for photographing a zero-level base line for the 
duration of the record. This level should cor¬ 
respond to the zero level indicated by the volt¬ 
age calibration. 

The sequence timer is to consist of ten relays 
operating in sequence with interlock. Each 
interval should be adjustable from zero to two 
seconds in ten steps. Neon bulbs should be 
provided for visual indication and the inter¬ 
locking device should prevent subsequent func¬ 
tions if any stage fails to operate. There should 
be provision for easy connection of external 
operations to the sequence timer. Each stage 
should have available two double-pole, double¬ 
throw sets of contacts. 

A muzzle-contact circuit should be provided 
which will supply a pulse adjustable in ampli¬ 
tude and duration applicable to the input of 
the amplifier. This circuit should work from 
either a make or break contact. 

A mica capacitor of 10 /if in ten equal steps 
should be supplied for each channel for use as 
ballast in piezoelectric measurements. Also, as 
mentioned under voltage calibration, a Gen¬ 
eral Radio decade capacitor or equivalent con¬ 
sisting of three banks of 0.001, 0.01, and 0.1 /if 
units should be provided for calibration. 

In addition to the circuits mentioned, a 
square-wave generator for visual check on 





124 


OSCILLOGRAPHS 


amplifier performance will be necessary. There 
must also be supplied a sweep circuit with 
necessary amplifiers and means for applying a 
sweep voltage to each of the tubes. A local 
oscillator, which will give a sinusoidal voltage 
from 20 c to 200 kc, is desired. 

There should be provided an intercommunica¬ 
tion system with two pickups without call-back 
feature, two 1,000-ft lines on reels, and a pub¬ 
lic-address system with a 50-w output from a 
blast-proof directional speaker with directional 
control within the trailer. 

An S-27 or Hallicrafter’s SX-28 receiver 
should be provided. Also, the trailer units 
should be furnished with an SCR-610 receiver 
to be supplied by the Ballistics Research Lab¬ 
oratory, Aberdeen. 

Voltage should be regulated either on the 
a-c mains or at the output of all the power 
supplies, so that calibration of the beam de¬ 
flections can be relied upon to one-half per cent. 

All relay racks should be grounded to the 
frame of the trailer. The circuits should be 
wired so that the chassis is not connected in 
any way to these circuits, but an insulated 
terminal should be provided on the rear of 
each chassis for a ground connection for the 
circuits. Also, there should be a terminal post 
on the frame of the trailer for a direct ground 
connection. 

Two 10-kw gasoline-driven motor-generator 
sets should be provided on a separate truck. 
Both should have the same electrical character¬ 
istics so that in the event of failure of one the 
alternate could be used on the same circuit. The 
truck of these motor-generator sets should be 
capable of moving the trailer in a limited man¬ 
ner, but not necessarily capable of towing the 
trailer over highways. For this reason both 
push and pull pintles should be provided. The 
truck should have two 30-gallon gasoline tanks 
and should be equipped with fire extinguishing 
equipment. Provision should also be made for 
operating the trailer unit on a-c mains of 110-v‘, 
single-phase, 60 c. The use of voltage-regulat¬ 
ing transformers is recommended in this con¬ 
nection. Provision should be made for easy 
switching between a-c mains and motor-gen¬ 
erator sets, and there should be precautions so 
that it is impossible to switch both power 
sources on at the same time. 


All components should be of first grade. 
Wire-wound resistors should be used wherever 
needed. Electrolytic capacitors should be avoid¬ 
ed throughout, and step-wise selector switches 
should be used in place of continuously variable 
potentiometers. The insulation must be of a 
type not affected by moisture. All wiring be¬ 
tween units should be in metal channels. The 
insulation should be at least 100 per cent over 
standard specifications. Transformers should 
be adequately shielded and insulated through¬ 
out. 

The trailer should be air conditioned with 
sufficient capacity to keep the relative humidity 
below 40 per cent under working conditions. 
Strip electric heaters are to be incorporated, so 
that the same blower and duct system can be 
used to distribute conditioned or heated air. 
This system should be supplied by the contractor. 

Dust covers are to be provided on all circuits 
where the accumulation of dust would be detri¬ 
mental to the operation of the unit. This ap¬ 
plies especially to relays and similar contact¬ 
ing devices. In this connection, it is suggested 
that a glass cover be provided for the cathode- 
ray oscilloscope panel and possibly a metal 
cover to be used in transit. 

A paper drier should be provided of the 
squirrel-cage or infrared types. 

The complete unit is to be housed in a K-72 
Army trailer, these units to be supplied by the 
Ballistics Research Laboratory. The contractor 
will make all necessary modifications. 

The trailer should contain a dark room for 
developing paper film with all necessary equip¬ 
ment such as safe lights, timing clock, cup¬ 
boards, Smith developing unit, fluid measures, 
thermometers, driers, etc. The sink shall be 
of stainless steel. There should also be included 
a 100-gallon copper water tank with external 
connections through the floor for filling and 
overflow. The tank should be piped to the sink 
with gravity feed and have a glass water-level 
gauge. A shop shall be provided with adequate 
and convenient a-c outlets and a work bench 
with drawers or cupboards for tools and small 
parts. 

There are to be twelve reels of cable beneath 
the trailer in weather-proof boxes, so mounted 
that each reel can be wound conveniently and 
independently of the others. Each reel should be 



MOBILE MULTI-CHANNEL CATHODE-RAY OSCILLOGRAPH 


125 


provided with a receptacle so that the input jack 
can be connected after the cable is unreeled. 
Signal Corps reels DR-5 and DR-4 are suggested. 
Each cable should be 1,000 ft long. The cables 
will consist of the following: four coaxial cables; 
four twisted-pair shielded cables for the inputs; 
two twisted-pair intercommunication cables; one 
twisted-pair trigger line; one twisted-pair muz¬ 
zle-contact line. These cables are to be rubber 
covered if possible. It is suggested that one of 
the communication lines be a twisted pair of 
No. 14 copper wire for alternative use as an a-c 
power line. There shall also be 200-ft lengths 
of rubber-covered cable for connection between 
the motor-generator truck and the trailer unit. 
It is desirable to have remote starting control 
for the motor-generator sets in the trailer. 

The construction of the whole unit should be 
as rugged and durable as is consistent with de¬ 
sign considerations. All sensitive circuits or 
mechanical assemblies should be completely 
anti-shock mounted. 

The trailer should be provided with fire ex¬ 
tinguishing equipment, first-aid kits, hand tools 
for electronic work, a 3-in. cathode-ray oscil¬ 
loscope, an RCA junior volt-ohmyst or equiva¬ 
lent, and a Simpson volt-ohm-meter or equiva¬ 
lent. 

44-2 Summary of Development 

A four-channel cathode-ray oscillograph com¬ 
plete with high-speed strip-paper recording 
camera, associated sequence-timer circuits, 
power supplies, sweep circuits, and control cir¬ 
cuits, was developed, constructed, and installed 
in a K-72 four-wheel Army trailer with asso¬ 
ciated gasoline-driven prime-power sources in¬ 
stalled in a K-53 Army tractor truck. The two 
units comprised a system available for field use 
for the recording of various ballistics measure¬ 
ments utilizing strain gauges of the piezoelec¬ 
tric and wire-resistance types of input signals. 
To facilitate field use, the trailers were also 
equipped with intercommunication systems, 
high-power public-address systems, and two- 
way radio communication. Dark rooms, air- 
conditioning, heating, work shops, and test 
instruments were also supplied for mainte¬ 
nance of the apparatus and its operation dur¬ 
ing the usual variations of weather in the 


vicinity of the Aberdeen or Dahlgren Proving 
Grounds. 

The four intelligence channels of the oscil¬ 
lographs consist of four d-c amplifiers designed 
by representatives of the Aberdeen Proving 
Ground and adapted to the operation of 9-in. 
cathode-ray tubes with a total accelerating po¬ 
tential of 10,000 v. The wiring diagram of the 
d-c amplifier is shown in Figure 12. The four 
tubes are mounted on a single panel with their 
centers at corners of a 60-degree equilateral 
parallelogram, so that they can be photo¬ 
graphed by the single lens of the strip-paper 
camera. Timing marks are also recorded by 
this camera simultaneously with the recording 
of the cathode-ray traces. This is done by use 
of flasher lamps driven by appropriate oscil¬ 
lator circuits, the lamps being located on the 
same panel and in the same focal plane as the 
cathode-ray traces. 

There are sweep circuits of the usual saw¬ 
tooth variety for study of the operation of each 
tube prior to photographic recording. However, 
during the recording period, the time axis is 
provided by the moving paper rather than by 
an electronic sweep circuit. 

A sequence timer is provided for timing of 
initiation of the field phenomena, starting of 
the camera, increase in cathode-ray tube illu¬ 
mination, automatic calibration, and so forth. 
This makes it possible to record the necessary 
information on a relatively short piece of pa¬ 
per, even at extremely high paper speeds. The 
general arrangement of the apparatus is shown 
in Figure 13. 

The camera utilizes 70-mm paper, obtainable 
in 200-ft rolls, and is capable of producing 
speeds from 10 to 500 in. per second, with a 
number of steps in speed. The camera opera¬ 
tion is almost completely automatic. After the 
appropriate record length has been selected and 
the paper cut off within the camera by the opera¬ 
tion of proper controls, the motion of the paper 
past the lens is begun by the sequence timer. 
Following exposure, the paper is delivered di¬ 
rectly to the dark room through a light-tight 
chute for immediate development. The camera 
is so designed that cutting off the paper is fol¬ 
lowed by automatic threading of the paper 
between the friction-drive rolls, so that no re¬ 
threading for the next run is necessary. This 



126 


OSCILLOGRAPHS 



Figure 12. Putnam d-c amplifier. 


makes a rapid series of runs possible. Inter¬ 
locking controls are provided to avoid faulty 
operation of various units. Acceleration of the 
camera to even maximum recording speed is 
accomplished in less than a millisecond without 
damage to the paper. 

The trailer and tractor truck are supplied 
with reels to hold various cables for trans¬ 
mitting intelligence signals to and from the 
truck for intercommunication and for con¬ 
nection of the two gasoline-driven motor-gen¬ 
erator sets to the trailer circuits. Thus, the 
complete assembly consists of a trailer and 
truck unit, self-powered, with adequate ap¬ 
paratus to allow the taking of records contain¬ 
ing frequency characteristics from 0 to 100 kc. 

In addition to the development of this partic¬ 
ular unit, which is in process of duplication for 
use by the Navy at Dahlgren Proving Ground, 
a similar, less elaborate system, mounted on 
dollies, was under construction for use within 
the laboratory only. This latter system con¬ 
tained all the main features described above, 
except the separate power supply, the trailer, 


the truck, and the dark room and work shop 
facilities. 

4 4 3 Description and Technical Information 

The physical characteristics and require¬ 
ments of this apparatus are adequately outlined 
by means of the military requirements and 
specifications noted above, the drawing of the 
interior trailer arrangement (Figure 13) and 
the description in Section 4.4.2. The actual 
amplifier circuit used to drive the cathode-ray 
tube is one designed for the Aberdeen Proving 
Ground, as has been previously stated, and is 
known as the Putnam amplifier. The basic cir¬ 
cuit is given in Figure 12. Certain minor modi¬ 
fications were made to adapt the amplifier to 
the operation of the 10-kw cathode-ray tubes. 
The following comments on this amplifier cir¬ 
cuit may prove of interest. 

The effective input impedance is approximately 100 
megohms, since the input tubes are operated at a stable 
grid potential point, i.e., where the free grid potential is 
approximately 0 v. No d-c return need be provided in the 
amplifier circuit, the actual return being effected by leak¬ 
age paths in cables attached to the amplifier during tests. 



























































MOBILE MULTI-CHANNEL CATHODE-RAY OSCILLOGRAPH 


127 



Figure 13. Mobile oscillograph interior. 


The amplifier can be used either single ended or push pull, 
merely by shorting out one grid, since the third stage 
automatically acts as a phase inverter. The response of 
the amplifier is essentially flat from d-c to 70 kc. It is 
only down about 20 per cent at 100 kc. The maximum out¬ 
put voltage is approximately 600 v peak to peak. If pre¬ 
aged tubes are used in setting up the amplifier for opera¬ 
tion, it is quite stable over long periods of time. The 
usual method for aging tubes is to apply normal heater- 
voltage and space-current conditions for 100 hours 
continuously prior to installation. 

The sequence timer consists of ten four-pole 
double-throw latch-type relays, used in the 
plate circuits of thyratrons. The controlled 
contacts are adaptable by means of patch 
panels to control any of the various items 
which must be synchronized, such as camera 
starting, tube-beam intensification, gun or 
bomb detonation, and calibration. The sequence 
timer can be set to have a range of timing 
intervals between the various channels from 
less than 0.02 second to 2.0 seconds in ten 
steps. It is based on variable RC time-delay 


circuits with interlocking control between the 
various channels. 

The pulse circuit, used to place timing marks 
on the paper, is comprised of a series of flasher 
tubes photographed in the same plane as the 
ends of cathode-ray tubes. These tubes are 
energized by a special arrangement of “flip- 
flop” circuits and countercircuits used to divide 
the output of a 100-kc crystal oscillator down 
to frequencies of 1,000, 500, 100, 50, and 10 c. 
If desired, calibration is provided automatically 
at the beginning of each record for charge, re¬ 
sistance, or voltage variations. This is effected 
by means of a spring-driven rotary switch, the 
contacts of which are so arranged that ten 
steps of calibration in addition to zero level for 
any of the three items noted can be introduced 
into any or all input circuits at the beginning 
of each record. The steps extend equally in a 
positive and negative direction and produce 
stair-step traces on the record to allow deter- 




























128 


OSCILLOGRAPHS 


mination of amplifier gain for accurate mea¬ 
surement. 

The camera operation can be understood by 
brief reference to Figure 14. While this dia¬ 
gram is oversimplified, the operation of the 
camera might be outlined as follows. Paper is 
unwound from the storage roll, S, and stored 



in a storage bin (not shown). The paper is 
precut to length, which makes it unnecessary 
to bring the storage roll up to speed. When the 
camera is started (prior to actual starting of 
the film or paper motion), drive roll A is run¬ 
ning at a preselected speed. The end of the 
paper rests between the separated drive rolls 
A and B, protruding just slightly beyond the 
line of contact. To start the film, B is pressed 
against the paper and the action of the rollers 
then pulls the paper through the camera. At the 
end of the run, roll B springs back, thus stop¬ 
ping the drive while the paper is delivered di¬ 
rectly from the camera via a stainless-steel 
chute into a dark room. The action of B is con¬ 
trolled by a powerful solenoid which gives 
practically instantaneous acceleration, since the 
inertia of the drive system connected to roller 
A is low and the power of the motor is suffi¬ 
cient to overcome the load imposed by the paper 
and drive roll B. Actually, B is geared to drive 


A in such a fashion that it is also rotating 
prior to contacting of the two rolls. The camera 
operation has been found to be quite satis¬ 
factory. 

The complete assembly is supplied with pow¬ 
er from two 10-kw generators driven by gaso¬ 
line engines mounted in a K-53 truck. These 
generators are automatically regulated by 
means of the speed control on the gasoline en¬ 
gine. The output is 110 v, 60 c, which makes 
it possible to operate the complete equipment 
without the truck when it is in the vicinity of 
commercial power source. 

Various auxiliary circuits are supplied, as 
specified in the military requirements, to allow 
the appropriate introduction and interpretation 
of signals. In addition, test equipment, consist¬ 
ing of a square-wave generator, a sinusoidal 
oscillator, voltmeters, and analyzers, is avail¬ 
able as part of the work room contained at the 
rear end of the trailer. A radio receiver is pro¬ 
vided for checking the timing-pulse-generator 
frequency against standard frequencies derived 
from Radio Station WWV. In addition, as pre¬ 
viously stated, intercommunication apparatus, 
high-powered public-address apparatus with a 
rotary externally mounted speaker, and two- 
way radio telephone are provided to maintain 
contact with central laboratory facilities, air¬ 
craft in bombing tests, and personnel working 
in the field outside of the trailer. 

45 ARMY AIR FORCES INSTRUMENT 
TRAILER (AC-67) 10 

Military Requirements 

The required military characteristics were 
as follows. 

1. The apparatus was to record the time of 
occurrence for a minimum of seven events. 

2. It was to measure and record intervals of 
time as follows. 

a. from 0 to 1 minute, with an accuracy of 
0.001 second; 

b. from 0 to 1 hour, with an accuracy of 
0.1 second; 

c. intermediate periods of time if (a) and 
(b) could not be met. 

3. The trailer was to have a source of power 
for operation of the apparatus. 


O 





ARMY AIR FORCES INSTRUMENT TRAILER 


129 


4. There was to be a radio for communica¬ 
tion and signal-transfer purposes with the con¬ 
trol station at Eglin Field and also with air¬ 
planes in flight. (It was later suggested that 
a special signaling device operating from bomb 
or trigger switches through the regular radio 
equipment should generate the signal to be 
picked up by the ground station in the instru¬ 
ment trailer. The special signaling device 
should fit into any airplane, require less than 
% hour for installation, require no specialized 
personnel, and operate on both very high and 
standard frequencies.) 

5. There was to be provision for heat for 
cold-weather operation and for ventilation for 
hot weather. (The completed trailer has a fan 
for ventilation purposes.) 

6. There was to be provision for control of 
humidity to a degree sufficient to protect the 
installed apparatus. (There is no provision for 
humidity control in the completed trailer.) 

7. The trailer was to be of sufficient size to 
accommodate the installed apparatus and to 
provide working space for the operating per¬ 
sonnel ; trailer construction was immaterial 
provided it had sufficient wheel bearing area 
to enable it to be hauled over sand roads. 

In conferences held after initial study of the 
problem it was agreed that the contractor 
should supply and install certain specific equip¬ 
ment : a multi-channel, multi-speed oscillo¬ 
graph; adjustable input channels for signals of 
varied strength; appropriate amplifiers for use 
with geophones, hydrophones, and photocells; 
set of geophones and hydrophones (12 each) 
(or microphones, should they prove superior to 
geophones) ; apparatus for signaling occur¬ 
rence of an event from plane to trailer; facili¬ 
ties for developing paper; and two 5-in. cath¬ 
ode-ray oscilloscopes. In addition to providing 
this equipment, the contractor was to under¬ 
take experiments to determine the practicality 
of using light-sensitive cells to determine time 
of burst for on-the-ground and low-altitude ex¬ 
plosions, the cells’ pickup to be useful at dis¬ 
tances greater than 500 ft. Suitable changes 
were to be made in the designated equipment 
in line with the results of these experiments. 

The Army Air Forces Proving Ground Com¬ 
mand agreed to furnish radio transmitters and 


receivers; office-type trailer for housing all the 
equipment; 120 ampere-hour batteries, 24-28 
v; 1-kw energizer; and a 28-d-c to 110-a-c 250- 
w inverter. 

Summary of Development 

The completed instrument trailer has a 24- 
element galvanometer and associated recording 
equipment, which provides for recording a con¬ 
siderably larger number of events than was 



Figure 15. Rear view of trailer showing terminal panel 
and radio antennas. 


originally requested. For a desired accuracy 
of 0.001 second the recording-paper speed must 
be about 25 cm per second, which, for an inter¬ 
val of 30 to 60 seconds, would give an impossibly 
long record. However, such accuracy is usually 
needed only during a small portion of a run; 
and, if the operator is forewarned, as he ordi¬ 
narily will be, of the portions requiring such ac¬ 
curacy, the operation can be carried out at vari¬ 
able speeds in such a way as to provide the re¬ 
quired accuracy during a run of one minute. 
With available paper speeds, the longest possi¬ 
ble run would be about 3 minutes; however, the 
recording galvanometer is of such versatility 
that moderate modification would meet the re- 
















130 


OSCILLOGRAPHS 



Figure 16. Interior view of rear end of trailer showing instrun ent installation. 


quirement for recording from 0 to 1 hour with 
an accuracy of 0.1 second. In addition, an ap¬ 
propriate and acceptable method for signaling 
from plane to trailer was developed and the 
necessary apparatus supplementary to the op¬ 
eration of the trailer was supplied. 

40-3 Description and Technical Information 

Figure 15 is a photograph of the exterior of 
the trailer with the radio antennas and the 
input panel mounted on the rear. Figure 16 is 
an interior view of the rear end of the trailer 
showing the instrument installation. Storage 
batteries are provided as the fundamental 
source of power. 

The most important item of equipment is the 
oscillograph, which consists of a 24-element 
galvanometer, motor-driven multiple-speed 
camera, control circuit, power circuit, and tim¬ 


ing equipment. The camera has paper speeds 
of 5, 12.5, 25, 50, 125, and 250 cm per second, 
and is designed so that at any of these speeds 
the paper speed is independent of the quantity 
of paper on the dispensing or receiving roll. 
Speeds as high as 350 cm per second can be 
obtained, but above 250 cm per second the 
speed control is poor. The camera uses 200-ft 
rolls of paper 10, 20, or 30 cm in width. 

The construction of the multi-element mov¬ 
ing-coil galvanometer is illustrated diagram- 
matically in Figure 17. The galvanometer 
elements are in individual cylinders which can 
be supported in the steel racks, C. An inner 
pole piece is mounted within the cylinder; an 
outer pole piece is inserted in the walls of the 
cylinder and serves to make magnetic contact 
between the inner pole piece near the coil and 
the steel rack in which all the elements are 


(j.llililllHHIIII'ffW, 











ARMY AIR FORCES INSTRUMENT TRAILER 


131 


mounted and which is subject to the magnetic 
fields of the permanent magnets D attached to 
its back. Light from a common source is re¬ 
flected by prisms onto the mirror of each gal¬ 
vanometer element, and, from the mirrors, 


The timing device consists of a 60-cycle elin- 
var tuning fork and a slotted shield rotated 
by a synchronous motor driven by the inter¬ 
rupted current from the fork. The accuracy is 
about 1 in 10,000 over long periods of time. 



Figure 17. Diagram illustrating construction of multi-element moving-coil galvanometer. 


passes through an optical system to the record¬ 
ing paper. 

The resistance of the galvanometer element 
is about 20 ohms. Its natural frequency is 110 
to 120 c; mechanical shock will cause the coil 
to vibrate slightly at about 230 c. A direct cur¬ 
rent of 1 microampere causes a light-spot de¬ 
flection of 1.5 to 2 mm. Besides the normal 
galvanometer elements two groups of 6 high- 
frequency galvanometer elements are also in¬ 
cluded ; one group, with peak sensitivity around 
1,800 c and a d-c sensitivity of 1.2 mm per mil- 
liampere; the other, with a peak sensitivity 
around 2,800 c and a d-c sensitivity of 0.65 mm 
per milliampere. At frequencies above their 
peaks these elements have approximately the 
same sensitivities as do the low-frequency ele¬ 
ments for the same frequencies. 


Four types of timing marks are possible: 0.01- 
and 0.1-second fine lines across the paper and 
1- and 5-second U-shaped timer deflection 
marks which can be imposed on any galvanom¬ 
eter trace. For very slow recording the 0.01- 
second lines can be blacked out. 

Recording can be done at two speeds, “record 
normal” and “record slow,” at the option of 
the operator at any instant. Assume that it is 
desired to record and determine the time be¬ 
tween two events, 25 to 35 seconds apart. A 
paper speed of 25 cm per second is used and 
the record normal button is pushed. The op¬ 
erator observes the traces in the viewing slot 
and, as soon as the first event occurs, the rec¬ 
ord slow button is pushed which reduces the 
paper speed to about 5 cm per second. Shortly 
before the second event is due the operator 





































































































OSCILLOGRAPHS 


132 


pushes the record normal button and the event 
is recorded at suitable speed. The timing mech¬ 
anism is operating during the entire record, 
with the result that the time determinations in 
the high-speed portions of the recording are 
unambiguous and are as accurate as though 
high speed had been used throughout the run. 

It was originally planned to use light-sensi¬ 
tive cells to determine the time of burst for 
both on-the-ground and low-altitude explosions. 
In the method developed for this purpose, the 
light flash is picked up by photocells and re¬ 
corded by the galvanometer after amplification. 
The amplifier is designed to be insensitive to 
slow changes in current due to changing light 
intensities resulting from clouds or other 
causes and thus gives a clear record of an ex¬ 
plosion. A barrier-layer or generating-type cell 
is used and is effective at 2,000 ft or less. Ex¬ 
periments with spotting charges M1A1, M-3, 
and M-4 modified indicate that there is a lag 
of about 0.003 second between the detonation of 
the shot-gun shell in the charge and the burst¬ 
ing of the can. There are certain applications 
(other than sound ranging) where this might 
have to be borne in mind. 

In connection with this light-sensitive device, 
microphones are used for seismographic rang¬ 
ing. Field amplifiers are placed at the observ¬ 
ing stations, connected to the photocells and 
microphones, and wired to the trailer by field 
wires. A compact input box at the trailer per¬ 
mits the operator to use the field wires for 
telephonic communication with any observation 
station, or to check whether the telephone, 
photocell amplifier, or microphone amplifier is 
on a given line. 

In later work the bombs being tested at Eglin 
Field were so increased in size that it was de¬ 
cided to drop them without explosives. When 
this was done, photocells and microphones were 
no longer usable since there was neither light 
nor sufficient noise. Geophones are used to 
register ground waves. These were installed at 
one of the test grounds and proved satisfactory 
for seismographic ranging in the absence of 
an explosion. The necessary techniques have 
therefore been devised for seismographic rang¬ 
ing with or without explosion. 

The occurrence of an event in a plane is 


signaled from plane to trailer by means of a 
momentary interruption of a 1,000-c tone being 
transmitted over the plane radio. Installation 
of the necessary apparatus in a plane is simple 
and the use of the apparatus does not inter¬ 
fere with the normal use of the radio for com¬ 
munication. Seismographic experience has 
shown that the time of stopping a tone can be 
read accurately through interference or static 
which would make voice communication virtu¬ 
ally impossible. The signal indicates either the 
“make” or “break” of a circuit. The opposite 
operation—closing a break circuit or opening a 
make circuit—gives no signal; hence, in using 
the signal it is immaterial whether the opera¬ 
tion is momentary or permanent. After the 
tone has stopped for about 0.05 second, it grad¬ 
ually returns to its former amplitude, ready 
to indicate another make or break very shortly 
after the one from which it has recovered. 

With this apparatus there was submitted a 
simple but adequate discussion of the principles 
of seismographic ranging, together with sam¬ 
ple calculations made by least squares. A meth¬ 
od of correcting for nonlinear propagation of 
sound in the immediate vicinity of an explosion 
was suggested. In addition there were supplied 
two very useful tables. One relates the velocity 
of sound in air to the temperature and the rela¬ 
tive humidity. The other gives the value of a 
function involved in seismographic ranging for 
a wide range of variables. This table greatly 
reduces the work involved in computation of 
specific problems. 



Figure 18. Range layout for experimental record shown 
in Figure 19. 


Figure 18 shows an arrangement of the ap¬ 
paratus for a simulated field test. A micro¬ 
phone, a photocell, and an amplifier were placed 
at each of stations 1, 2, 3, and 4. The trailer 
was located midway between 1 and 2. Purely 















PHOTOCELL STATION 1 


\RMY AIR FORCES INSTRUMENT TRAILER 


133 



a l 


Figure 19. Specimen record taken with trailer equipment. 

















































































































































































































































































134 


OSCILLOGRAPHS 


for test purposes the signal transmitter was 
placed 13 miles distant from the trailer. Its 
record is shown in the 9th trace, labeled “Radio 
Signal,” on Figure 19. The first 8 traces re¬ 
cord the signals from the photocells and micro¬ 
phones, the 10th, the record made by the timer. 
The 11th trace shows the breaking of a wire 
wrapped around the can by the exploding charge. 

The charge was to be fired some 20 seconds 
after the second tone break which is shown at 
1.758 seconds. After this break the record slow 
button was pushed; and a return was made to 
record normal about 20 seconds later. At 21.692 


seconds the charge exploded, as is shown by 
photocell flashes PI, P2, PS, and P4, and by 
trace 11. Arrival of the sound at the micro¬ 
phones is indicated by Ml, M2, M3, and M4, the 
times being respectively 22.907, 23.417, 22.823, 
and 23.438 seconds. In this connection, one of 
the points requiring greatest care in the analy¬ 
sis of a record is the determination of the 
initial microphone breaks. Calculations made 
with these data indicated the shot to have oc¬ 
curred at x = -(-0.3 ft, y = -f-1296.6 ft based up¬ 
on station 3 as the origin of coordinates. The sur¬ 
veyed location was x = 0.0 ft, y = +1298.0 ft. 



Chapter 5 


HIGH-VOLTAGE X-RAY RADIOGRAPHY a 


By John A. Hornbeck b 


INTRODUCTION 

WO PROJECTS for the investigation of X-ray 
radiography with extremely high voltages 
were initiated by Section D3, Instruments, of 
NDRC, one at the Massachusetts Institute of 
Technology [MIT] in the summer of 1941, and 
the other at the University of Illinois in Janu¬ 
ary 1942. When these investigations were be¬ 
gun, high-voltage radiography in the range up 
to one million volts was gaining an established 
place in the production of ships, tanks, guns, 
planes, and other war materiel. The researches 
in higher voltage ranges were undertaken be¬ 
cause it appeared that they might offer im¬ 
portant advantages for radiography of objects 
of medium and large thicknesses (greater than 
4 or 5 in. in equivalent steel thickness). Thus, 
the investigations were begun to ascertain the 
radiographic possibilities of extremely pene¬ 
trating X-rays and to develop techniques for 
their use. 

Each investigation was built around a spe¬ 
cific machine or device for generating high- 
voltage X-rays. At the University of Illinois 
this device was the betatron, which had been 
designed and developed previously by D. W. 
Kerst. At MIT it was the electrostatic gener¬ 
ator, which had been developed previously by 
R. J. Van de Graaff. Each of these machines 
constitutes a method of accelerating electrons 
to very high velocities. X-rays are electromag¬ 
netic radiations like visible light but of very 
much shorter wavelength and are produced by 
the collision of these high-energy electrons with 
a metal target. X-ray radiographs, or pictures, 
are made when a beam of X-rays passes 
through a dense object (such as a metal cast¬ 
ing) and impinges upon a photographic film. 
In passage through the object the X-ray beam 
undergoes selective absorption, i.e., the thicker 
portions of the object absorb more X-radiation 

a OD-148, NO-123. 

b Bell Telephone Laboratories. 


than do the thinner portions. Accordingly, on 
reaching the film the X-ray beam, which may 
be assumed to have been uniform originally, 
varies in intensity over its cross section. The 
detection of this variation of intensity by the 
photographic emulsion results in an X-ray 
radiograph. 

A betatron and its control panel are shown 
in Figures 1 and 2. A production-type betatron 
X-ray generator installation is shown in Fig¬ 
ure 3. In a betatron electrons are given energy 
by the accelerating effect of a changing mag¬ 
netic field. A toroidal (doughnut) vacuum tube 
is placed between circular, specially shaped 
pole pieces of a laminated, a-c magnet which 
is operated at a frequency of 180 c. Electrons 
are injected tangentially from a “gun,” which 
is placed within the tube, shortly after the 
magnetic field has passed through zero. As the 
field increases the electrons are accelerated 
around the tube, gaining an average of about 65 



Figure 1. 20-MEV betatron magnet. 

135 








136 


HIGH-VOLTAGE X-RAY RADIOGRAPHY 



Figure 2. Betatron control panel. 


electron volts [EV] of energy on each revolution 
in a 20-million electron volt [MEV] machine. 
In obtaining their high energies the electrons 
make tens of thousands of revolutions through 
the tube in the one-quarter cycle during which 
the magnetic field is increasing. The magnetic 
field is so distributed that the electrons are 
focused into a very fine beam and brought to 
a circular path called an equilibrium orbit. 
After one-quarter cycle the magnetic field 
reaches its maximum, and the electrons attain 
their maximum energy. At this stage the field 
conditions that hold the electrons in an orbit 
are artificially upset, and the electrons are 
brought to the target where X-rays are pro¬ 
duced. The X-rays emerge from the doughnut 
in a narrow cone in the forward direction. Be¬ 
cause of the a-c excitation of the magnet the 
X-ray beam from a betatron is a pulsating one. 
In these two respects the betatron beam differs 
from that of the electrostatic generator. In the 
case of the latter, X-rays are radiated through¬ 
out the entire solid angle about the target, 
i.e., 4 t r ; furthermore, the beam intensity is con¬ 
stant with time, i.e., not pulsating. 


The electrostatic generator, as operated for 
producing X-rays, is fundamentally a conveyor 
system for carrying negative charge from the 
ground to an insulated conducting terminal or 
sphere. The difference in potential between the 
conducting sphere and ground depends upon 
the total charge deposited. This device is shown 
schematically in Figure 4. Figures 5 and 6 
are photographs of the open generator, the 
X-ray tube and the control panel. In practice, 
a motor-driven, endless belt of insulating ma¬ 
terial runs between a pulley inside the sphere 
and a pulley at ground potential. Negative 
charge is deposited on the moving belt by 
corona from a row of sharp needle points, 
which are maintained at a negative potential 
opposite the grounded pulley, the potential be¬ 
ing established usually by a transformer-recti¬ 
fier set. The belt carries the charge to the top 
pulley. There the charge is extracted by another 
set of needle points and flows to the conduct¬ 
ing sphere or terminal. The potential difference 
due to this charge accumulation is maintained 
across the electrodes of a special, highly in¬ 
sulating X-ray tube. This tube is conventional 
in the sense that it contains a heated filament 
which supplies the electrons that are ac¬ 
celerated toward the anode or target, the tube 
current depending solely on the thermionic 
emission at the temperature of the filament. 
X-rays are formed by the impact of the ac¬ 
celerated electrons on the gold target in the 



Figure 3. Production-type betatron X-ray generator 
installation. 








INTRODUCTION 


137 



COLUMN INSULATORS 


RAY TUBE 


INSULATING BELT 


EQUIPOTENTIAL RINGS 


ORIVING MOTOR 


GENERATING VOLTMETER 


HIGH VOLTAGE TERMINAL 


COOLING COIL 


FOCUSING MAGNET 


FILAMENT GENERATOR 


Scale m fecncs 


LEAD SHIELDING 


X-RAY TARGET 


Figure 4. High-voltage X-ray generator. 




















































































138 


HIGH-VOLTAGE X-RAY RADIOGRAPHY 



Figure 5. Electrostatic generator and X-ray tube. 


tube. In steady-state operation the charging 
current to the terminal (conducting sphere) 
equals the X-ray tube current plus leakage 0 
and corona losses between the high-potential 
parts and ground. In practice, the upper limit 
on the potential difference obtainable is set 
by the dielectric strength or insulation resist¬ 
ance between the high-voltage parts and 
ground. In order to improve the dielectric 
breakdown strength, compact generators are 
operated at a gas (usually air) pressure con¬ 
siderably above atmospheric (200 psi in the 
NDRC generators). 

c A controlled leakage is introduced for increased sta¬ 
bility and other purposes. 


The consequence of the use of two different 
means of generating high-voltage X-rays was 
that two quite different wavelength bands were 
studied by the NDRC contractors. With the 
betatron the voltage range examined was from 
3 to 20 million volts [MV], whereas with the 
electrostatic generator the voltage range was 
from 0.5 MV to 2.5 MV. The interactions be¬ 
tween matter and the X-rays differed for the 
two voltage ranges. In the range up to 2.5 MV, 
the two most important X-ray absorption 
processes are the Compton effect and the photo¬ 
electric effect. Pair production does not enter 
the picture below 1.0 MV, and it is still a small 
effect at 2.5 MV. At higher voltages, however, 
pair production becomes rapidly more impor¬ 
tant, with the consequence that iron, for ex¬ 
ample, is least opaque to X-ray quanta of 
about 7-MEV energy. It should be apparent 



Figure 6. Control panel for electrostatic generator. 

























INTRODUCTION 


139 


that radiography in the voltage ranges men¬ 
tioned here is quite different from that car¬ 
ried on at ordinary commercial voltages (say 
200 to 400 thousand volts) where the photo¬ 
electric effect is predominant. 

It is important to remember that the X-rays 
produced by any generator have a continuous 
distribution in energy. Two characteristics of 
the spectral distribution should be noted par¬ 
ticularly. (1) The highest radiation energy 
produced is definite, corresponding to the volt¬ 
age across the X-ray tube, i.e., to the maximum 
energy of the electrons striking the target. The 
well-known equation relating tube voltage, V, 
and the maximum X-ray energy E max is 

eV=E max , (1) 

where e is the electronic charge. The energy of 
an X-ray quantum is related to frequency and 
wavelength by the following expression: 

E = hv = h C j (2) 

A 

where h is Planck’s constant, \ is wavelength, 
and c is the velocity of light. From the above 
equations it may be shown that the tube voltage 
by fixing the maximum energy determines a 
maximum X-ray frequency (v mai = eF/ft) and 
a corresponding minimum wavelength (\ min 
= hc/eV). (2) Although all energies up to the 
maximum are present, the maximum intensity 
is concentrated in an energy band which 
occurs at approximately %V max . The practical 
significance of this is that a radiographic image 
results more from this important band of 
energies than from the X-rays of highest 
energy. 

In all radiographs, irrespective of the volt¬ 
age of the incident beam, the clarity of the 
image in the photographic emulsion is reduced 
by the effect of secondary or scattered radia¬ 
tion. In each of the three processes by which 
high-energy X-rays are absorbed (photoelectric 
effect, Compton effect, and pair-production) 
secondary X-rays and/or electrons are pro¬ 
duced which are always of less energy than 
the primary quanta of radiation. The secon¬ 
daries move off, in general, at an angle to the 
direction of the primary rays and therefore 
tend to blur the radiographic image formed 
by the transmitted primary or direct rays. 


This scattered radiation varies with the energy 
of the primary quanta and with the type of 
absorption process involved. In general, the 
higher the primary energy the more nearly 
straightforward the secondary radiations move, 
and hence the less deleterious the effect of the 
scattered radiation. This statement is com¬ 
pletely at variance with the recommended tech¬ 
niques for low-voltage radiography, in which 
the lowest voltage possible is recommended as 
giving the least effect from scattered radiation. 
Both statements are true and compatible be¬ 
cause: (1) in low-voltage radiography the 
photoelectric effect is predominant and the 
scattering decreases with tube voltage; (2) at 
higher voltage the other effects come into play 
and the net result of the three is that the scat¬ 
tering decreases with increasing voltage. It is 
therefore obvious that as the voltage of the in¬ 
cident X-rays is increased the harmful effects 
of scattering pass through a maximum at some 
voltage which is known to lie somewhere be¬ 
tween 0.5 and 1.0 MV. 

The three absorption effects enter in such a 
way that for each material there exists an opti¬ 
mum voltage for penetration, i.e., least opaque¬ 
ness. This is one reason why there is an upper 
voltage limit for practical radiography. As the 
primary energy is increased beyond the ab¬ 
sorption minimum some of the secondary or 
tertiary scattered X-rays become more pene¬ 
trating than the primary. Less clarity of image 
results when this effect becomes too strong. 
From the investigations undertaken, a 20-MV 
X-ray source does not appear to have reached 
the upper limit which seems likely to be at 30 
to 40 MV. The optimum tube voltage will prob¬ 
ably vary also with the atomic number of the 
specimen, since the voltage for minimum ab¬ 
sorption varies with this quantity, being higher 
for steel than for aluminum. 

In everyday practical radiography it is re¬ 
quired to obtain a good-quality radiograph in a 
reasonable exposure time. For a given thick¬ 
ness of specimen, the exposure time is gov¬ 
erned by the X-ray output of the generator and 
the voltage at which it is operating. The clarity 
of the radiograph may be expressed in terms 
of a number of variables: latitude, definition, 
sensitivity, and contrast. These factors, in turn, 



140 


HIGH-VOLTAGE X-RAY RADIOGRAPHY 


depend upon the type of X-ray film used and 
its processing, the voltage of the incident X-ray 
beam, the physical size of the focal spot, the 
geometry of the specimen, and the geometry 
of the experimental setup (i.e., target-specimen 
distance and specimen-film distance). It was 
the plan of the NDRC projects to evaluate 
quantitatively the effects of these variables on 
the quality of the radiograph and the exposure 
time within the voltage ranges mentioned. If 
the advantages resulting from the use of high 
voltages for radiography were significant, it 
was further planned to design and construct 
generators which could be used in the war pro¬ 
gram. As a result of the NDRC work, the tech¬ 
niques of high-voltage radiography were thor¬ 
oughly examined and a number of machines of 
both types were delivered to and placed in use 
by the Armed Services. 

52 ADVANTAGES OF HIGH VOLTAGE 
FOR X-RAY RADIOGRAPHY 

An examination of the factors involved in 
radiography, mentioned above, brings out the 
advantages of high-voltage X-rays. 

The radiographic method is employed for the 
examination of flaws within an object or speci¬ 
men. The ability of a radiographic technique 
to do this for a particular object is measured 
by the smallest flaw which can be observed. 
This is expressed quantitatively by sensitivity, 
defined to mean the minimum percentage 
change which is observable in the total thick¬ 
ness of an object. For example, a 2 per cent 
sensitivity means that a flaw equal in thickness 
to 2 per cent of the total object thickness is 
just discernible. A smaller flaw is not observed. 
Thus loiv sensitivity means good sensitivity. 
Another important measure of the quality of a 
radiograph is the definition, defined to mean the 
ability to perceive detail. Although definition 
and sensitivity are related, it does not follow 
that if the sensitivity is good (low in value) 
the definition is sharp, or vice versa. When 
both the sensitivity and definition have opti¬ 
mum values the radiograph is of good quality. 
In addition, practical considerations demand 
that the exposure time be of a short, control¬ 
lable length, and that the number of exposures 


needed to radiograph an object completely be 
as small as possible. This latter requirement 
depends upon the thickness which can be ex¬ 
amined significantly by one exposure for a 
given exposure time. This thickness is called 
the latitude. 

5 21 Exposure Time 

The exposure time t and the exposure E are 
determined by the X-ray tube current, i, the 
decrease of the intensity of the incident radia¬ 
tion with increasing steel thickness (a function 
of X-ray energy), the intensity of the incident 
radiation, and the sensitivity of the detection 
instrument. In this case the detection instru¬ 
ment is the emulsion of the photographic film. 
Of the variables mentioned, the one principally 
important to exposure time is the decrease of 
intensity of the incident radiation with in¬ 
creasing steel thickness. (This statement will 
be qualified later.) This is a consequence of 
the exponential absorption of X-rays by mat¬ 
ter, the equation for which is 

I 9 = I 0 c-**. (3) 

In the above, I 0 — the intensity of the incident 
X-ray beam; I= the intensity of the same 
X-ray beam after it has passed x units of 
length along its path through the metal or 
specimen; p, = the absorption coefficient of the 
material involved; and e — the base of the 
Naperian logarithm system. 

The diminution of intensity for a given 
thickness of material traversed depends sensi¬ 
tively upon /I. In general, p, is the sum of three 
components: the photoelectric effect, the Comp¬ 
ton effect, and pair-production. p, varies with 
the energy of the incident beam and the atomic 
number of the absorbing material. It decreases 
rapidly from 0.5 MEV d to the neighborhood of 
4 MEV, depending on the absorbing material. 
Beyond this point the change is less rapid with 
increasing voltage. The practical significance 
of the exponential absorption is shown by the 
following illustration. Suppose the problem is 
to radiograph a steel forging which is 14 in. 
thick. The actual exposure time required for 

(1 MEV is a unit of energy and it therefore refers to a 
particular X-ray frequency or wavelength. MV is a unit 
of potential difference. Thus a 2-MV generator produces 
X-rays of energy up to 2 MEV. 




ADVANTAGES OF HIGH VOLTAGE FOR X-RAY RADIOGRAPHY 


141 


making this radiograph with an electrostatic 
generator operating at 2 MV and 0.4-ma tube 
current was 4 hours. At 1 MV under otherwise 
similar conditions, the exposure time would 
have been 12 weeks, and at 0.5 MV the exposure 
time would have been about 500 years. With 
200 milligrams of radium under these condi¬ 
tions, computations indicate than an exposure 
of about 5 years would be required. On the 
other hand, at 4 MV the calculation indicates 
that the exposure required for this forging 
would be about 1 minute. For X-rays produced 
by a 20-MV generator this exposure time would 
be slightly shorter, assuming the same initial 
X-ray intensity. (The incident intensity of X 
rays from a 20-MV betatron is less than that 
obtainable with existing electrostatic genera¬ 
tors.) For radiographing iron and steel sec¬ 
tions, the biggest gain is obtained by using 
voltages up to at least 4 MV. The practical 
consequence of this is that for radiography of 
heavy metal sections (8 to 16 or 20 in. of 
steel) the employment of high-voltage X-rays 
makes the difference between a practical and an 
impractical exposure time. In the example 
given it is clear that even at 1 MV, the prewar 
industrial limit, the exposure time would be 
far too long. 

Both the betatron, operated at 20 MV, and 
the electrostatic generator, operated at 2 or 
2.5 MV, give reasonable exposure times for 
thick metal sections. For the thickest metal 
sections, i.e., 12 in. and above, the 20-MV 
machine will give exposure times substantially 
less than those of the 2-MV machine. Actually 
exposure times for the two devices are about 
the same for 6 in. of steel, viz., 20 seconds. 
For thinner steel sections the electrostatic gen¬ 
erator is faster, although this is of little prac¬ 
tical importance since the exposure time for 
6 in. is very short. For thicker sections the 
20-MV betatron is faster. This comparison is 
made on the basis of an electrostatic generator 
operated with 0.3-ma tube current, and a 20- 
MV betatron with about 0.1-/xa current. In both 
cases the target-film distance is 24 in., the film 
Eastman Industrial Type A developed 8 minutes 
at 68 F to a density of 1 above fog density. 
It should be noted that practical differences in 
exposure time occur only for steel thicknesses 


above 10 in., inasmuch as for this thickness 
the exposure time with the electrostatic gen¬ 
erator, using Industrial Type A film, is a rea¬ 
sonable period of approximately 15 minutes, 
whereas that for the 20-MV betatron is ap¬ 
proximately 2 minutes. The fact that for some 
steel thicknesses the low-voltage machine has a 
shorter exposure time than the high is readily 
explained by introducing another of the vari¬ 
ables mentioned above, viz., X-ray output or 
incident intensity. The electrostatic generator 
may supply an X-ray tube current perhaps 300 
or more times that obtainable with the beta¬ 
tron. Even though the production of X-rays is 
more efficient at 20 MV than at 2 MV, it still 
takes 5 or 6 in. of steel to permit the superior 
absorption coefficient for the higher-voltage 
radiation to take over. 

Latitude 

The latitude is determined by the decrease in 
the intensity of the radiation when the object 
thickness is increased, and by the end points of 
the useful density range on the film. This quan¬ 
tity is quite complicated, in that it varies con¬ 
siderably with the choice of film and with the 
choice of the system for viewing the film. For 
example, a high-intensity viewing system in¬ 
creases the useful density range of most X-ray 
film. It has been established, however, that the 
latitude in the range 0.5 to 2 MV increases with 
increasing voltage, being perhaps 80 per cent 
greater at the higher voltage. No comparable 
analysis has been made for the voltage range 
from 3 to 20 MV. The available evidence indi¬ 
cates that at 20 MV variations of 6 in. of steel 
thickness could be radiographed without sig¬ 
nificant loss of detail. 

5 2 3 Definition 

The definition is measured by the diameter of 
the disk, or circle of confusion, forming on the 
film the image of a point on the surface of the 
object or specimen facing the X-ray source. For 
a given radiographic setup this diameter is 
completely determined, except for film charac¬ 
teristics, by the size of the X-ray source, the 
cross-sectional diameter of which is usually re¬ 
ferred to as the spot size. From the geometry 






142 


HIGH-VOLTAGE X-RAY RADIOGRAPHY 


of a radiographic setup it follows that 
Definition = Spot size X 
Distance from the top of object film 
Distance to top of object from X-ray source 

Since the definition is directly proportional to 
spot size, good definition demands an extremely 
small spot size. In both the betatron and the 
electrostatic generator spot sizes of maximum 
dimension no more than 0.010 in. were achieved 
in practice. This spot size may be compared 
with that of 0.25 in. available in commercial 
high-voltage X-ray generators (e.g., the res¬ 
onant-transformer type). 

The other factor upon which definition de¬ 
pends is the size of the area of the photographic 
film which is sensitized by a single X-ray quan¬ 
tum. Film grain size, of course, varies enor¬ 
mously with the result that the sensitized area 
may be smaller than the grain size of the film 
(though only for a small focal spot). Hence, 
the grain size essentially determines the resolu¬ 
tion of the film and, therefore, sets the lower 
limit on definition. 

It should be noted here that small focal-spot 
size and high definition are not exclusively at¬ 
tributes of high-voltage radiography. The focal- 
spot size depends upon the electron optics of 
the X-ray tube. It is easier to obtain a focused 
beam when there is not a wide spread in the 
energies of the electrons comprising the beam. 
Thus, in general, it is usually easier to focus 
a beam accelerated by a constant-potential 
source, such as an electrostatic generator, than 
one accelerated by a variable-potential source. 

Sensitivity and Scattering 

Sensitivity, as previously defined, is related 
to the smallest density difference perceptible on 
the film. This difference must correspond to a 
change in the direct radiation component of the 
total radiation; i.e., density differences due to 
scattered radiation are excluded. In fact, scat¬ 
tering increases the sensitivity (makes it 
worse). At 2 MV the sensitivity decreases from 
approximately 0.50 per cent at 3 in. of steel 
to 0.41 per cent at 14 in. of steel. Over this 
range of steel thickness the sensitivity de¬ 
creases faster than a straight line drawn be¬ 
tween the end points mentioned above. The 


sensitivities at 0.5 MV and at 1.0 MV are ap¬ 
proximately the same functions of steel thick¬ 
ness, decreasing from approximately 0.54 per 
cent at 3 in. to 0.47 per cent at 14 in. Thus at 
some voltage between 0.5 and 1.0 MV the sen¬ 
sitivity passes through a maximum (i.e., has 
its worst value). If a means is found (this will 
be discussed later) for preventing the scat¬ 
tered radiation from reaching the film, the sen¬ 
sitivity increases linearly with voltage. Thus, 
when scattering is eliminated, one should work 
with as low a voltage as is feasible. When scat¬ 
tering is present, the sensitivity also depends 
rather critically on the geometry of the radio- 
graphic setup. Thus with one radiographic 
specimen it may be inherently impossible to 
obtain as good quality using good techniques 
as it is with other geometries. 

With the 20-MV X-ray machine it has been 
found that the minimum detectable thickness 
is almost independent of object thickness and 
of the position of the flaw within the object. 
Thus, at this voltage the sensitivity decreases 
(becomes better) linearly with increasing ob¬ 
ject thickness. This is a consequence of the 
fact that at this high voltage there is relatively 
little scattered, or secondary, radiation reach¬ 
ing the photographic plate. Quantitatively, at 
this voltage with Type A film, a 1 per cent sen¬ 
sitivity has been obtained at 3 in. of steel thick¬ 
ness, and a 0.21 per cent sensitivity at 14 in. 
thickness. 

In terms of scattering the advantages of the 
20-MV radiation over that of lower voltages 
may be expressed quantitatively by a figure of 
merit called the “scattering factor.” The scat¬ 
tering factor is defined as the ratio of the scat¬ 
tered radiation present to the radiation in 
the direct beam at any given depth of penetra¬ 
tion. After passing through 4 in. of steel the 
scattering factor is 0.2 for 20 MEV, 0.7 for 
10, 4.0 at 2, 7.0 at 1, and even larger at 0.5 
MEV. The experimental method of measuring 
the scattering factor at 10 and 20 MEV is 
open to some question, however. 

There are practical ways (e.g., use of the 
Potter-Bucky diaphragm and “backing away”) 
of reducing the adverse effect of scattered ra¬ 
diation in some voltage ranges, but only at the 
expense of increased exposure time. In the 





SUMMARY OF WORK 


143 


radiography of especially thick objects, ex¬ 
posure time requirements preclude the use of 
these techniques. Where exposure time was not 
a limitation, use of a Potter-Bucky diaphragm 
(illustrated in Figures 7 and 8) yielded amaz¬ 
ingly good sensitivities at 2 MV by reduction 
of scattered radiation, particularly in setups 
where the geometry of the object was such that 
the scattering effect was pronounced. A similar 
diaphragm designed for use with a 20-MV 
generator was found to offer no improvement 
in scattering factor. This may be attributed to 
(1) the fact that less scattered radiation is 



present at the higher voltage and (2) the fact 
that radiation which is scattered at the higher 
voltage is deviated through such small angles 
that it cannot be easily removed by a Potter- 
Bucky diaphragm. 

s.3 SUMMARY OF WORK 

5-31 X-Ray Radiography Work 

with the Betatron* 

In January 1942 work was started on the 
study of the radiographic properties of X-rays 

e OD-148. 


in the energy range between 3 and 20 MEV. 
Such X-rays were readily produced by the 
betatron, and the main initial purpose of the 
work was to ascertain and improve the value 



Figure 8. Potter-Bucky diaphragm. 


of the betatron for practical radiography. Work 
with the betatron was continued until July 30, 
1945. 1 - 18 

The work performed can be subdivided into 
2 main headings: the design and improvement 
of the betatron for radiographic purposes, and 
the investigation and development of radio- 
graphic properties and techniques of high- 
voltage X-ray radiation. The major results of 
the work may be classified in 5 groups. 

1. From a study of the nature of high-energy 
radiation and of the radiographic character¬ 
istics of the betatron, it has been established 
that the 20-MV betatron is useful for a wide 
range of radiographic problems and that its 
X-rays are exceptional in radiographic quality 
and speed for thick metal sections (6 to 20 in.). 
This part of the work included a study of the 
high-energy X-ray characteristics of many 
types of film. 

2. A number of important improvements 
have been made in the 20-MV betatron which 
have increased the X-ray yield, have improved 
the mechanical and electrical stability and have 
simplified operation and maintenance of the 
machines. A 4-MV machine which is portable 
and quite simple to operate has been designed. 

3. At the University of Illinois three com¬ 
plete machines of the 4-MV size were built, of 
which two were put in use elsewhere. Two 













144 


HIGH-VOLTAGE X-RAY RADIOGRAPHY 


20-MV machines were completed and put into 
operation. These were manufactured largely by 
the Allis - Chalmers Manufacturing Company, 
Milwaukee, Wisconsin, under an NDRC con¬ 
tract, according to designs and specifications 
drawn up with the help of University of Il¬ 
linois personnel. The University of Illinois 
personnel installed and tested the 20-MV 
machines as radiographic units. They also co¬ 
operated with the Naval Research Laboratory, 
Anacostia, D. C., in the manufacture and ini¬ 
tial operation of a third 20-MV machine. 

4. Sealed-off glass and porcelain betatron 
vacuum tubes, or doughnuts, in sizes suitable 
for the 4-MV and 20-MV betatrons were de¬ 
veloped and small-scale manufacturing prob¬ 
lems in connection with them were solved. 

5. The original betatron at the University 
of Illinois has been applied to radiography of 
numerous specimens submitted by the NDRC 
and other government agencies. 

532 X-Ray Radiography Work with the 
Electrostatic Generator f 

Work with the use of the electrostatic gen¬ 
erator for high-voltage radiography was be¬ 
gun in the summer of 1941 and was concluded 
in June 1945. 19 ' 23 The initial purpose of this 
project was to investigate the radiographic 
quality of high-voltage X-rays, to develop tech¬ 
niques for the use of a 2-MV electrostatic gen¬ 
erator and to prepare designs of such a genera¬ 
tor suitable for Service and industrial use. Five 
complete generators (four at the expense of 
the Navy) were constructed under NDRC con¬ 
tract. The first of these units was installed at a 
Navy field station in May 1943. A second was 
shipped to another Navy station in November 
1943. The third unit was completed shortly 
thereafter, but was not shipped until March 
1945 because of a delay in building construc¬ 
tion at the Navy station to which it was as¬ 
signed. A fourth unit has been delivered to 
the Navy. In accordance with the purpose for 
which it was planned, the fifth unit was used 
at MIT for testing and development work in 
connection with the operation and improve¬ 
ment of the other four units. 

In addition to the design work there was un- 

f NO-123. 


dertaken a very intensive experimental and 
theoretical investigation of the radiographic 
properties of X-ray radiation in the voltage 
range from 0.5 to 2.5 MV. A quantitative exami¬ 
nation of the increase of X-ray exposure with 
steel thickness at various voltages was made 
with the use of X-ray film and with sensitive 
radiation-detection devices. The scattering of 
high-voltage X-rays was studied from both 
theoretical and experimental points of view, 
with the result that clear understanding of the 
phenomena has been obtained. From this re¬ 
search has come the development of radio- 
graphic techniques to minimize the fogging ef¬ 
fect of scattered radiation on the film. In the 
exploration of 2-MV radiography and in the 
research-development of new radiographic 
techniques for use in the field, about 5,000 
radiographs were made. These were not only 
of test objects, but also of a large variety of 
objects of special radiographic interest. Some 
of these were of prime importance to the war 
effort and were sent from various places to 
MIT because they could not be radiographed 
with lower-voltage equipment available else¬ 
where. One of the most important develop¬ 
ments associated with the project has been the 
X-ray tube built by the Machlett Laboratories, 
Inc., with collaboration by MIT in design and 
testing. Although the tubes now in use are 
continuously evacuated by a pumping system 
a sealed-off tube was supplied by Machlett 
toward the end of the project, which success¬ 
fully passed various tests including 50 hours 
of operation at 2 MV. Extensive research on 
various other aspects of the generator was con¬ 
ducted to improve the performance of the ma¬ 
chines in the field. Attempts were made to im¬ 
prove the insulating charging belt, the resis¬ 
tors, the insulators, and to test the use of gases 
other than compressed air (e.g., SF C ) for in¬ 
sulation. 

54 EVALUATION 

These high-voltage radiography projects 
made a real and important contribution to the 
war effort. The delivery of high-voltage X-ray 
machines to the Army and Navy is one indica¬ 
tion. The first 20-MV betatron constructed by 
Allis - Chalmers Manufacturing Company was 


mMBUKMaT, 




EVALUATION 


145 


delivered to the Manhattan Project in the sum¬ 
mer of 1944. A second, similar machine was 
delivered to the Rock Island Arsenal, Rock 
Island, Ill., in February 1945. The first 2-MV 
electrostatic generator was delivered to the 
Bureau of Ordnance, Navy Department, in the 
spring of 1943, and has been in almost con¬ 
tinuous use since then at their Explosives In¬ 
vestigation Laboratory, Stump Neck, Md. The 
second electrostatic generator has been in serv¬ 
ice on the West Coast since the end of 1943. 
In addition, the NDRC contractors operated 
their generators on occasion for radiographing 
war equipment. 

These investigations have important applica¬ 
tions for postwar industry inasmuch as they 
have opened up, for the first time, the field of 
X-ray radiography above 1 MV. The advan¬ 
tages of the use of voltages above this figure 
have already been pointed out. Not only have 
machines been constructed and demonstrated to 
be practical for industrial usage, but the theory 
and experimental techniques involved in high- 
voltage radiography have been worked out in 
a commendable fashion. 

The following general statements can be 
made concerning the use of the betatron and 
the electrostatic generator for radiographic 
purposes. There are certain basic advantages 
connected with the use of the higher-voltage 
betatron. The radiation produced by the large 
betatron is better for radiography principally 
because there is less scattering associated with 
it, because wider film latitudes result and be¬ 
cause somewhat thicker specimens can be ra¬ 
diographed with reasonable exposure times. 


Since the betatron is mechanically simpler than 
the electrostatic generator there is probably 
less chance of a breakdown interfering with 
the operation of the former. However, an elec¬ 
trostatic generator operating at 4 MV (the 
higher voltage being made possible by use of 
a better insulating gas) would probably be 
superior to the 20-MV betatron in exposure 
time for even thick sections. Tentative designs 
for this higher-voltage electrostatic generator 
were worked out under the NDRC contract. In 
evaluating the projects the most important 
point to be kept in mind is that both machines 
are definitely superior to available commercial 
apparatus in regard to both quality of radiog¬ 
raphy and exposure time. 

Practical application of the small 4-MV beta¬ 
trons (one of which was delivered to the British 
and the others to the Navy) is open to some 
question. Their X-ray output is small and does 
not compare with the outputs available from 
the 20-MV betatron and the 2-MV electrostatic 
generator. For the radiography of some metal 
sections not over 5 in. thick they may have ap¬ 
plication, since up to this thickness the ex¬ 
posure times are not excessive (one hour for 
4 in. of steel), and the quality of the radiograph 
should be better than that obtained with lower- 
voltage equipment with relatively large focal 
spots. 

Both these projects carried on extensive in¬ 
vestigations of the theory and application of 
high-voltage radiography. These researches re¬ 
sulted in information of importance not only 
in radiography and applied physics but also in 
pure scientific knowledge. 




Chapter 6 


BOMB INSTRUMENTATION ' " 

By F. L. Yost c 


INTRODUCTION 

HE IMPORTANCE of air warfare made it 
necessary to improve bombing tables and 
to extend them to cover high-altitude bombing. 
Aberdeen Proving Ground, which was con¬ 
cerned with making such bombing tables avail¬ 
able to the Army Air Forces and which was 
conducting an extensive experimental program 
on bombing, requested the development of a de¬ 
vice which would indicate the retardation ex¬ 
perienced by a bomb during its flight. 3 During 
the development of this device, Aberdeen Prov¬ 
ing Ground further requested 13 that an inves¬ 
tigation be made of the feasibility of using 
standard seismic detectors for determining the 
time and position of bomb impact (no explosion 
involved) on the ground in connection with 
range bombing. 

62 MILITARY REQUIREMENTS 

The original request 3 was for the develop¬ 
ment of two instruments. One was to deter¬ 
mine the time of impact of a falling bomb on a 
time scale coordinated with its time of release 
from the plane. It was suggested that the de¬ 
vice should consist of a low-powered high- 
frequency radio transmitter mounted in the 
bomb, a switch for turning on the transmitter 
shortly after the time of release and a receiv¬ 
ing apparatus for converting the signal into 
suitable form for recording on an oscillograph. 
The other device was to measure the retarda¬ 
tion of a falling bomb due to air resistance. 
It was suggested that the device consist of a 
one-component accelerometer to be mounted in 
the bomb and so designed that its readings 
could be transmitted by radio to a ground sta¬ 
tion. No specification was made of the ac¬ 
curacies desired in these instruments. How¬ 
ever, as they were needed to supply data ap- 

a OD-90. 

^ OD-124. 

c Technical Aide, Division 17, NDRC. 


plicable to the computation of bomb trajec¬ 
tories, especially for bombs released at high 
elevations, it was clear that as high accuracies 
as possible were desired. Actually, one instru¬ 
ment was developed which sufficed for both re¬ 
quirements. 

The request 13 for studying applications of 
seismograph equipment for determining time 
and position of bomb impact was not specific as 
to results expected. It merely asked that the 
possibilities be investigated. 

63 SUMMARY OF DEVELOPMENT 

The work on this project was done by the 
Gulf Research and Development Company. For 
determining the retardation of a falling bomb, 
there was designed for installation in it a small 
accelerometer and a radio transmitter j 14 ’ 6 ’ 7 
the combination transmits to a ground station 
an audio frequency dependent upon the re¬ 
tardation being experienced by the bomb. The 
mean density of the unit is very close to the 
density of the explosive it replaces, and thus it 
does not alter the normal ballistics of the 
bomb. Units are made in two lengths: one for 
installation in either 500-lb or 1,000-lb bombs 
and the other for 2,000-lb or 4,000-lb bombs. 
This allows two standard sizes of units to ac¬ 
commodate four sizes of bombs without the 
accelerometers being more than 11 in. from 
the center of mass in any case. 

In flight a bomb has a slight vertical angle 
of attack between its axis and the tangent to 
its trajectory, and undergoes oscillations in 
pitch and yaw about its center of mass. An 
accelerometer mounted at the bomb’s center of 
mass and aligned with its axis indicates not the 
total retardation, but rather the retardation 
multiplied by the cosine of the angle the axis 
makes with the tangent to the trajectory. In its 
final design, the apparatus permits measure¬ 
ment of bomb retardation continuously during 
flight to an accuracy of about 1 per cent of g 



146 



DESCRIPTION AND TECHNICAL INFORMATION 


147 


through the range 0 to g and measurement of 
the time of flight to 5 milliseconds. 

The contractor’s personnel assisted the Bal¬ 
listic Research Laboratory, Aberdeen Proving 
Ground, in making a number of drop tests with 
4,000-lb bombs, from which the drag functions 
suitable for construction of bombing tables 
could be deduced. The Ordnance Department 
procured from the contractor approximately 
30 units for various sizes of bombs to use in 
connection with other bomb-ballistic studies. 
The method of computation of the trajectories 
from these data was worked out in the Ballistic 
Research Laboratory. Among the interesting re¬ 
sults from these studies was the observation of 
the centrifugal forces introduced into the accel¬ 
erometer by the yawing motion of the bomb. 
These studies also gave opportunity for observa¬ 
tion of the magnitude of the dispersion (varia¬ 
tion between bombs) in drag forces. It is ex¬ 
pected that these accelerometer units will con¬ 
tinue to be useful tools in bombing studies. 

The contractor not only specified suitable 
seismographic instrumentation, as a result of 
tests, but also outlined the general methods of 
handling the information in deducing the time 
and position of the impact of a bomb in range 
bombing. 5 ' 8 As a result, a seismographic de¬ 
tector installation was procured for the Muroc, 
Calif., bombing range and for two years it has 
been the mainstay of the instrumentation for 
determining time of flight. 

64 DESCRIPTION AND TECHNICAL 
INFORMATION 

6-41 Bomb-Retardation Device 

Figure 1 is a block diagram showing the 
method used for measuring axial retardation 
of the bomb. Retardation is the difference be¬ 
tween g and the actual acceleration of the 
bomb. The accelerometer generates an audio 
frequency (750 to 900 c) which is changed by 
the accelerating forces acting upon a suspended 
mass. This audio frequency modulates the sig¬ 
nal of a radio transmitter (70 me) which is 
radiated from an antenna composed of the 
bomb and a wire whip extending beyond the 
tail. The signal is detected and amplified in a 
commercial ultra-high-frequency radio receiver 


on the ground. It is then filtered to remove in¬ 
terference and mixed with a fixed-frequency au¬ 
dio signal to produce a low beat frequency. This 
beat frequency is recorded on a photographic 
tape controlled by a chronograph, from which 
the original audio frequency at any instant may 
be computed. 



Figure 1. Block diagram showing method used for 
measuring axial retardation of bomb. 


Subtraction of any beat frequency from that 
corresponding to zero retardation gives the fre¬ 
quency change resulting from the retardation; 
and the value of the retardation is determined 
from the calibration curve of frequency change 
vs retardation. A beat frequency is used be¬ 
cause low frequencies are more easily recorded 
on a chronograph than high frequencies. The 
higher frequencies generated in the accelerom¬ 
eter are more easily amplified and filtered 
and they are also far enough above the 60-c 
recorder power and the 100- to 300-c micro¬ 
phonics of the radio tubes in the bomb to keep 
interference at a minimum. 

To serve as an accelerometer, a steel plunger 
is suspended in the coils of a Hartley oscilla¬ 
tor by spiral springs at either end. The springs 
permit motion of the plunger, relative to the 
coils, parallel to its own axis and the axes of 
the springs themselves; but they offer con¬ 
siderable stiffness to motion transverse to that 
direction. The springs have a linear stress- 
strain relationship through % in. of motion. 
The accelerometer is so mounted that the axis 
of the plunger is parallel to the axis of the 
bomb. Regardless of the attitude of the bomb 
when falling, the only thing which can cause a 
distortion of the springs is the retardation ex¬ 
perienced by the bomb, the component of re¬ 
tardation along the axis of the bomb being the 
only part effective in causing distortion. For 
retardations involving accelerations from 0 to 
g the displacement of the plunger and the cor- 















148 


BOMB INSTRUMENTATION 


responding change in frequency of the Hartley 
oscillator are proportional to the retardation 
experienced. The accelerometer is arbitrarily 
built so that an increase in retardation causes 
a decrease in frequency. 

The accelerometer transmitter is built around 
two 1G6 GT double triode tubes, as shown in 
Figure 2. One triode section is used for the 


antenna coil in the tank-coil field receives power 
from the tank circuit and, through a coaxial 
transmission line, delivers it to the whip and 
bomb. 

The retardation-measuring unit is built with 
a large threaded bushing at the base of the 
antenna post, which fits into the fuse hole of 
.standard tail plates. A large nut is screwed up 


1G6-GT 

AUDIO BUFFER 

OSCILLATOR 


I G6-GT 

R-F OSCILLATOR 



Figure 2. Wiring diagram of accelerometer transmitter. 


Hartley oscillator accelerometer. Its plate is 
transformer-coupled to the grid of the second 
triode, which comprises the buffer stage. The 
other two triodes are connected in push-pull in 
an r-f tank circuit to form an r-f oscillator. The 
plate of the buffer stage is transformer-coupled 
to modulate the grids of the r-f oscillator. An 


tightly to secure it in place. This type of 
mounting allows units to be adjusted and cali¬ 
brated before shipping; then at the bomb-load¬ 
ing shed they can be placed inside the bomb and 
packed in sand before the tail plate is attached. 
The design of this unit has all the advantages 
noted in the first paragraph of Section 6.3. 






















































DESCRIPTION AND TECHNICAL INFORMATION 


149 


In order to determine the retardation asso¬ 
ciated with a given frequency change, the ac¬ 
celerometer must be calibrated. This is done 
by noting its audio frequencies when its axis 
is inclined at known angles to the horizontal. 
If 8 is the angle between the accelerometer 
axis and the horizontal, the accelerating force 
/ acting against the spring is / = ma = mg sin 
8', and hence the “retardation” being experi¬ 
enced is g sin 8. Any retardation between — g 
8 and g may be obtained by proper selection 
of the angle 8. In calibration there is some¬ 
times a slight bowing of the curve of frequency 
change vs retardation due to transverse motion 
of the plunger. To correct for this, frequency 
readings are taken for steps of 0.1# from —0.1 g 
to -f< 7 , both up and down the scale. The unit 
is then rotated 180 degrees about its longitu¬ 
dinal axis and another similar set of readings 
is taken. The frequencies recorded for each ac¬ 
celeration position are averaged and plotted on 
graph paper against the corresponding acceler¬ 
ation or deceleration. The calibration graph for 
a normal unit is a straight line which passes 
through the origin, r.s shown in Figure 3. 

In order to be certain that this method of 
averaging does not introduce or mask possible 
errors, an independent determination of fre¬ 
quency change vs retardation from 0 to g was 
made in a manner which involved no trans¬ 
verse loading (i.e., by a beam-loading method 
with vertical motion of the plunger). The pro¬ 
cedures were found to be equivalent. 

When data on retardations are collected it is 
the beat frequencies corresponding to various 
positions of the plunger which are recorded. 
If the calibration curve for the unit is known 
and if the beat frequency of some known ac¬ 
celeration can be recorded just prior to or dur¬ 
ing flight, the true retardation may be com¬ 
puted. In the final design of the accelerometer 
unit a solenoid was incorporated to pull the 
plunger against a fixed stop for zero calibra¬ 
tion. The frequency difference F k between the 
fixed-stop position and the zero retardation is 
determined by calibration; for example, F k for 
Figure 3 is 22.5 c. During a test the beat fre¬ 
quency of the fixed-stop position is recorded 
just prior to release of the bomb. 

Figure 4 is an assembly drawing of the ac- 



retardation in percent of gravity 

Figure 3. Calibration graph for normal unit. 

celerometer, showing the solenoid arrangement. 
When the solenoid is energized by a 6-v battery 
external to the bomb, the armature is pulled to 
the right until the head of the plunger engages 
the zero-calibration stop; when the solenoid is 
not energized, the armature is returned to 
neutral by a coil spring. The neutral position 
of the armature is toward the nose of the bomb, 
so retardation forces tend to aid the spring 
action rather than oppose it, thus preventing 
the armature from interfering with the motion 
of the accelerometer plunger during flight. The 
solenoid is attached to a brass plate the pur¬ 
pose of which is to conduct heat from the sole¬ 
noid to the steel case. Without the plate there 
is danger that the hexane, which is used as a 
damping fluid in the plunger chamber, will boil, 
rupturing the housing, if the solenoid is ener¬ 
gized for more than 10 minutes at a time. 

The equipment which records the bomb’s re¬ 
tardation is designed to give a continuous rec¬ 
ord of the signal transmitted by the accelerom¬ 
eter unit when a bomb is released at high 
altitude. The axial component of the retarda¬ 
tion of the bomb can be found by measuring 
the change in frequency of the audio signal; the 
time of impact with the surface of the earth can 
be found by determining the time at which the 
transmitter signal fails; and the time of flight 
can be found by comparing the time of impact 





150 


BOMB INSTRUMENTATION 



with the time of a signal given when the bomb is 
released. It is necessary to have communication 
with the plane so that the equipment may be 
turned on and adjusted before the bomb is re¬ 
leased. Figure 5 is a block diagram of the record¬ 
ing equipment. 

The output of the Variac goes to the Halli- 
crafter receiver, the 180-v power supply, the 
135-v power supply, and the charger. If the 
plane transmits a signal at the time the bomb 
is released, the National 1-10 release channel 
receives the signal and indicates the time of 
release by driving an oscillograph element in 
the photographic recorder. The National 1-10 
time-pulse channel serves to calibrate the tim¬ 
ing lines on the photographic tape so that the 
time of flight of the bomb may be accurately 
determined; it also allows the record to be 
correlated with other tests being made on the 
bomb. The Hallicrafter S-27 receives the signal 
transmitted by the retardation unit in the 
bomb. The recorder amplifier converts the 
audio output of the S-27 at about 850 c into a 


beat frequency of 25 to 125 c for retardation 
and into a direct current for impact. The syn¬ 
chronous timing-line motor is driven by the 
output of a tuning-fork amplifier. 

The photographic recorder has six oscillo¬ 
graph elements having a natural frequency of 
200 c and a sensitivity of about 2 mm per mil- 
liampere, mounted in pairs in double-element 
boxes; it has a seventh element having a natu¬ 
ral frequency of about 44 c and a sensitivity of 
about 30 mm per milliampere, in a single-ele¬ 
ment box, for high sensitivity. The camera 
holds 3 31/32-in. seismograph recording paper 
and is equipped with a magazine and a collec¬ 
tor each holding 200 lineal ft. A paper speed 
of 8 to 15 in. per second may be used. 

Figure 6 shows a portion of a bomb-retarda¬ 
tion record. The average beat frequency at cer¬ 
tain intervals is determined from such a re¬ 
cord by an appropriate method of differences. 
These beat frequencies together with the cali¬ 
bration constants of the unit and the zero-cali¬ 
bration correction permit plotting the retarda- 































































































































































DESCRIPTION AND TECHNICAL INFORMATION 


151 


TO 



Figure 5 . Block diagram of recording equipment. 


tion as a function of the time, as is done in 
Figure 7. 

The curves in Figure 7 are for two different 
tests. In plotting the curves the retardation of 
each unit was determined at nine evenly spaced 
points in each second and the curves were 
drawn through the plotted points. The smooth¬ 
er of the two curves is for an accelerometer 
near the center of mass; the more ragged curve 
is for a test in which the accelerometer was at¬ 
tached to the bomb plate about 42 in. from the 
center of mass, and shows the effect of cen¬ 
trifugal accelerations resulting from pitch and 
yaw superimposed on the retardation. 

Seismographic Instrumentation 
and Technique 

The seismographic tests showed that the 
ground characteristics of the new bombing 
field at Aberdeen are ideal for bomb-impact 


and distance measurements. The seismic wave 
generated by a 100-lb bomb dropped from 2,000 
ft is sufficient to give a good “arrival” 500 ft 
from the impact point. The noise from the 
carrying plane registers on the detector either 
through the air or through the ground distur¬ 
bance set up by the sound waves. Obviously 
bomb records must be above this noise level. On 
the basis of observed noise from the experi¬ 
mental plane the following table of minimum 
bomb size for good arrivals on a detector 500 
ft from the impact was computed. 


Height of 

Minimum 

Approximate nitro- 

plane 

bomb size 

starcb equivalent 

2,000 ft 

100 lb 

3 1b 

5,000 ft 

20 lb 

1.51b 


Although it is quite possible to measure bomb 
impacts under more adverse conditions by read¬ 
ing events one half or one cycle late and apply- 






































































152 


BOMB INSTRUMENTATION 


TIMING LINES FROM lOO'CYCLE FORK. 

1. ... ..I-T* 


TIME IN SECONDS 


24.4 24.5 24.6 24.7 24.8 24.9 25.0 25.1 




l t t t I < t • M t I 


(H M#I| mXm • ««M I |X< i 1(01 n ill II I«t 


zr.~t? 



TIME PULSE' 



Figure 6. Portion of bomb-retardation record. 


ing a uniform correction, the accuracy suffers 
considerably. An accuracy of 0.002 second in 
the determination of impact time may be ob¬ 
tained in this area by the firing of nitrostarch 
(or dynamite) charges at the points of impact, 
or by a seismic calibration of the area. There 
being no outstanding differences in the behav¬ 
iors of seismic detectors with natural frequen¬ 
cies of 15 c and 30 c, it was recommended that 
from the standpoint of ruggedness 30-c de¬ 
tectors be used. 

For the location of the point of impact, some 
sort of spatial arrangement, or “grid,” of seis¬ 
mic detectors is necessary; and the arrival time 
of waves at not less than three detectors must 
be determined. The contractor’s reports'’' 8 give 
diagrams facilitating the calculation of impact 
locations for certain grid arrangements, and 
they discuss in some detail the degrees of ac¬ 
curacy which can be expected for different grid 
arrangements. In addition, they specify pro¬ 
cedure for seismic reconnaissance (or evalua¬ 
tion) of a range-bombing site and for seismic 
calibration of a range-target area. 

Tests were made in Chesapeake Bay to deter¬ 
mine whether seismic measurements of bomb 
impacts on water could be made by means of 


vertical-component detectors buried on the bot¬ 
tom. It was doubtful whether sufficient ac¬ 
curacy could be obtained to allow the use of 
seismic methods for determining location and 
the instant of impact on the water. Although 
the tests were not conclusive in that they did 
not include all of the available methods of 
measuring seismic arrivals, it seems apparent 
that a bomb does not generate a steep seismic 
wave front upon impact but transmits its 
energy more gradually to water than to ground. 
This will probably prevent measurement of 
impacts on water from being made with the 
same degree of accuracy as those on land. 

6-s HISTORY 

When work on this project was first started 
a number of methods of determining retarda¬ 
tion were considered. There were four possible 
schemes. 

1. To determine the change in pressure in a 
liquid under the retarding forces on the bomb. 

2. To determine the change in resistance of a 
resistance strain gauge with a suitable mass 
attached to it. 

3. To determine the change in inductance of 













IMPACT 41.388 SEC 


HISTORY 


153 




Figure 7. Plots of bomb retardations as functions of time. 


















154 


BOMB INSTRUMENTATION 


a coil due to a change in the reluctance of the 
magnetic circuit occasioned by the retarding 
forces on the bomb. 

4. To determine the change in capacitance of 
a condenser due to motion of its plates occa¬ 
sioned by the retardation forces on the bomb. 

No experimental work was carried out on 
schemes (1) and (4). Some rough tests were 
made on (2), but (3) looked so much more 
promising from preliminary tests that it was 
the one actually developed. 

From the beginning the development was 
straightforward, with modifications resulting 


from actual bomb tests conducted from time to 
time at the Aberdeen Proving Ground. For ex¬ 
ample, one such modification of the original 
plan was the inclusion of the solenoid arrange¬ 
ment for holding the plunger in a fixed posi¬ 
tion while a beat frequency was recorded just 
prior to release of the bomb. Before such an 
arrangement was adopted it was necessary to 
record the beat frequency when the bomb was 
in position in the plane and to determine the 
“effective retardation” associated with that 
beat frequency from the angle of inclination of 
the axis of the bomb. 



Chapter 7 


DEFLECTION-TIME MEASURING DEVICES 1 

By F. L. Yost b 


INTRODUCTION 

he U. S. Navy Bureau of Ships was con¬ 
ducting an extensive series of tests of dam¬ 
age from underwater explosions against simu¬ 
lated hull shapes and caissons, in order to evalu¬ 
ate the effectiveness of different types of struc¬ 
tures in resisting explosive forces. Some of these 
tests were made on full-scale and others on re¬ 
duced-scale models. Due to the cost of labor and 
materials in constructing caissons of the type 
used, it appeared to be essential that the utmost 
in useful information be derived from each test. 
The most useful measurements were those of 
deformations and deflections of various portions 
of a structure as functions of time plotted on the 
same axis. 

Prior to the development recorded here, mea¬ 
surements of initial movements in such tests 
had been obtained from velocity meters and ac¬ 
celerometers, the remaining movements being 
measured by deflection-indicating devices such 
as “streak” cameras, plastic strain gauges and 
banks of contactor points. None of these devices 
had the desirable feature of providing, on a sin¬ 
gle continuous record and consequently on one 
time base, complete information as to the veloc¬ 
ity and deflection (both positive and negative) 
of a given point on a test bulkhead. 

The Bureau of Ships therefore requested that 
a project be set up by the National Defense Re¬ 
search Committee for the purpose of developing 
devices for the measurement of displacement as 
a function of time. 

7-2 MILITARY REQUIREMENTS 

It was requested that two types of device be 
developed: (1) one not involving mechanical 
attachment to the moving target, and (2) one in¬ 
volving attachment to the target, provided only 
that this type offered substantial simplification 
over the former. 

a NS-197. 

b Technical Aide, Division 17, NDRC. 


The basic performance specifications were as 
follows. 

1. The device was to record deflections of from 
2 to 3 ft. 

2. The device was to provide a good record of 
the path of a point on the structure as a function 
of time after impact, even if the point attained 
velocities of 300 or more feet per second. 

3. Any device attached to the moving target 
was to withstand high initial accelerations (esti¬ 
mated to be possibly 2,000#), although it need 
not record the values of the accelerations. 

The most immediate need of the Bureau of 
Ships was for a device to use in connection with 
large-scale tests. In such tests the deflections 
encountered were of the order of 12 in. and the 
model was of such size that the attachment of 
a moving member to it did not detract appreci¬ 
ably from the accuracy of the measurements. 
Accordingly, the first type of unit developed 
was an electromagnetic deflection unit [EMU] 
which required the affixing of a member of the 
test object. 

Sometimes the tests conducted by the Bureau 
of Ships are on as small a scale as %c> that is, 
the gauge of the metal used is y 1(i full-size and 
the charge is suitably reduced. For tests on such 
a small scale, the metal is so light that attaching 
a test device to it would affect the results. Ac¬ 
cordingly, when the EMU was nearing com¬ 
pletion the Bureau of Ships asked that the con¬ 
tractor devote his efforts to the other part of the 
original request, namely the development of a 
device not requiring attachments to the test ob¬ 
ject or requiring attachments with so little iner¬ 
tia as to have no appreciable influence on the 
results from a model of considerably reduced 
scale. The optical deflection gauge [ODG] re¬ 
sulted from this phase of the work. 

The basic specifications for the ODG were as 
follows. 

1. A minimum of apparatus was to be attached 
to the structure under consideration and its na¬ 
ture to be such as not to alter in any appreciable 



155 



156 


DEFLECTION-TIME MEASURING DEVICES 


way the deflection of the test structure itself. 

2. The device was to record a deflection range 
of at least 15 in. 

3. The deflection was to be recorded to an 
accuracy of Ys in. over the 15-in. range. 

4. The device was to have stability and reli¬ 
ability of operation. 

73 SUMMARY OF DEVELOPMENT 

This project was conducted under contract 
with Faximile, Inc. The first apparatus de¬ 
veloped 1 was the EMU. With this device the 
maximum error introduced by changes in mag¬ 
netic environment during a test (which deter¬ 
mined the overall accuracy) was less than 5 per 
cent. Both positive and negative bulkhead move¬ 
ments were recorded on a linear time base in 
units of 100 microseconds. The actual time at 
which the charge was set off could be indicated 
directly on the record. Bulkhead velocity and ac¬ 
celeration could be calculated from the deflection 
record. Deflections of several points on the bulk¬ 
head could be recorded on the same time base 
by using two or more units. The control and re¬ 
cording apparatus could be installed at locations 
as far as 900 ft from the caisson under test. 
Three units of this type were supplied to the 
Bureau of Ships. 

The second device developed 2 was the ODG, 
which satisfactorily fulfilled the requirements 
imposed on its development. One unit of this 
type was supplied. 

74 DESCRIPTION AND TECHNICAL 

INFORMATION 

7-41 Electromagnetic Deflection Unit 

The EMU 1 (in conjunction with an oscillo¬ 
scope and recording camera) is an electronic 
device designed especially to measure the deflec¬ 
tion and velocity of a test bulkhead during the 
time it is undergoing the effects of an explosive 
charge. 

With the EMU, this is accomplished by mea¬ 
suring the coupling between two coils. One of 
them is mounted directly on the test bulkhead 
and is energized to produce a 10-kc electromag¬ 
netic field; the other is mounted opposite on the 
fixed bulkhead, or other stable mounting. It is 


aligned on the axis of the driver coil, and is used 
as a secondary coil in which a 10-kc voltage is 
induced. The induced voltage varies with the 
distance between the coils and its variation is 
used to measure that distance. 

Movements of the test bulkhead are thus re¬ 
solved into amplitude variations of a 10-kc sig¬ 
nal across the pickup coil. This signal is ampli¬ 
fied and applied directly to the deflection plates 
of an oscilloscope. The trace of the oscilloscope 
is photographed on a strip of 35-mm film mount¬ 
ed on a revolving drum the peripheral speed of 
which is such that measureable definition is 
given to the 10-kc signal. 

In recording the signal, the oscillogram is 
limited to an amplitude of 20 mm peak to peak. 
Due to the coarse grain of the highly sensitive 
film used, this limited amplitude of recording 
would result in poor definition of small signal 
variations, thus contributing to low sensitivity 
of the entire system. For example, in recording 
a 12-in. bulkhead deflection as a 0- to 20-mm 
amplitude on the film, it would be impossible to 
define on the record amplitudes corresponding 
to less than ^-in. deflection. However, by divid¬ 
ing the 12-in. deflection into three equal ranges 
and consecutively recording them as 0- to 
20-mm amplitudes, almost three times the or¬ 
iginal definition is obtained. The shift from one 
range to the next is automatic, and accomplished 
electronically. 

The schematic arrangement for a test is shown 
in Figure 1. The electric circuit diagram for the 
EMU is given as Figure 12 in the instruction 
manual which accompanies the original report 1 
of this device. The oscillator-amplifier section 
supplies a high-level source of stabilized 10-kc 
power to the driver coil which is cable-connected 
to its output. 

The driver coil is a center-tapped 2-mh in¬ 
ductance wound on a 3-in. form and requires a 
capacity of about 0.12 ^f to resonate at 10 kc. 
The pickup coil is a 1-mh center-tapped induc¬ 
tance wound on a 12-in. diameter form. In a test, 
the two coils are rigidly mounted on bulkheads 
so that the driver and pickup coils are on a com¬ 
mon axis and initially at a predetermined dis¬ 
tance from each other. 

Measurement of bulkhead deflection with the 
EMU consists basically of recording amplitude 





DESCRIPTION AND TECHNICAL INFORMATION 


157 


variation of the 10-kc signal induced in the pick¬ 
up coil as the coupling distance between the two 
coils varies. This record is interpreted by com¬ 
paring it against a calibration curve plotted 
from values of amplitude for unit steps of tbp 


CAPACITY UNIT 



PICKUP COIL 

Figure 1 . Schematic diagram of arrangement for test. 


coupling distance between coils. Curves of coup¬ 
ling characteristics made within test caissons 
were found to be closely logarithmic. These 
characteristics are subject to variations due to 
magnetic environment but, for the purpose of 
illustration, a logarithmic-coupling curve can be 
assumed as indicated in Figure 2. 

In recording amplitude variations of the 10-kc 
signal on film for any deflection range, it is 
necessary that the gain control of the ampli¬ 
fier in use be adjusted so that the maximum volt¬ 
age induced in the pickup coil (corresponding to 
the maximum deflection of any range) is re¬ 
solved into a 3-in. oscilloscope trace that may be 
photographed within the lateral limits of the 
35-mm film. 

In attempting to record a deflection of 18 in., 
from an initial spacing between coils of 36 in., 
the ratio of maximum to minimum voltage is 
very small at the beginning of the range and in¬ 
creases sharply toward the end. Recording a 
deflection in this manner would result in a very 


poor definition on the film of variations within 
the first half of the range. 

By making use of a separate amplifier channel 
for each 6-in. step of the assumed 18 in.—each 
amplifier incorporating an individual gain con¬ 
trol which can be adjusted to keep the oscillo¬ 
scope trace within the limits previously men¬ 
tioned—it becomes possible to secure almost 
three times the sensitivity and definition ob¬ 
tainable with a single-channel amplifier. Figure 
3 shows an oscillogram obtained with such an 
arrangement. Note that the 10-kc signal is suffi¬ 
ciently defined in the oscillogram that it can be 
used for the time basis. 

The three channels mentioned above are fed 
into a common output system so that only one 
oscilloscope is necessary to make the complete 
record. A trigger circuit consisting of two 
tetrode thyratrons and a dual triode is employed 
in order to switch each channel into operation 
only for the duration of the 6-in. step for which 
its gain control has been adjusted. During actual 
operation, the three amplifier channels are con¬ 
nected in parallel and the initial condition is such 
that only the first channel is in operation, while 
the remaining two are biased to cutoff. The ris¬ 
ing output voltage of the entire amplifier system 
is used to trigger a thyratron tube at the com¬ 
pletion of the first 6-in. step which, in turn, 
biases the first channel to cutoff and removes the 



INDUCED VOLTAGE ACROSS PICKUP COIL AT 10 KC 


Figure 2. Relation between voltage induced in pickup 
coil and distance between pickup and driver coils. 


high bias from the second channel, causing it to 
become conductive. The gain control of the sec¬ 
ond channel is adjusted for the second 6-in. step, 
at the completion of which the output voltage is 
again used to “fire” a second thyratron biasing 
the second channel to cutoff and causing the 























































































158 


DEFLECTION-TIME MEASURING DEVICES 



Figure 3. Typical EMU-3 deflection record. 


third channel to become conductive. In addition 
to all this, a threshold limiter is incorporated 
within the amplifier which can be adjusted to 
pass only those voltages above the initial level 
(i.e., no signal is recorded when the coils are at 
their initial separation). Also, the first and sec¬ 
ond and the second and third ranges are ar¬ 
ranged to overlap slightly so that both positive 
and negative bulkhead readings can be recorded 
during an explosion. 

In connection with recording the compara¬ 
tively large deflections of about 36 in., the initial 
movements of the bulkhead demand the greatest 
interest and require that the first two ranges of 
the EMU be adjusted for high sensitivity. This 
may be accomplished only by decreasing these 
ranges, with the result that the third range is 
also necessarily decreased, making it impossible 
to record the entire deflection over the three 
ranges. However, by incorporating an additional 
thyratron tube the function of which is to reduce 
the threshold value of the limiter stage for the 
duration of the third range, it becomes possible 
to record greater deflection distances within this 


range, since the voltage range of the third chan¬ 
nel is increased. The unit which does this is 
called the EMU-3B range expander unit [REU]. 

Optical Deflection Gauge 

After the development of the EMU and the 
REU, it was deemed desirable, for reasons al¬ 
ready mentioned, that the ODG 2 be developed. 
The operation of the device is based entirely 
upon optical principles, necessitating the attach¬ 
ment of a minimum of apparatus to the moving 
object under consideration. 

The ODG consists of two separate pieces, the 
light unit [LU] and the poiver unit [PU]. In a 
test arrangement, the LU and PU are supple¬ 
mented, as shown in Figure 4, by a voltage regu¬ 
lator, a recording amplifier, and a recording 
oscilloscope. 

The LU consists of two coaxial optical sys¬ 
tems, one for transmitting light and the other 
for receiving it. Essentially, the device measures 
the distance betwen the LU and a reflecting sur¬ 
face on the optical axis by measuring the mag- 








DESCRIPTION AND TECHNICAL INFORMATION 


159 



Figure 4. Arrangement of optical deflection gauge for 
testing structure. 


nitude of the reflected light received by the unit 
for a fixed magnitude of transmitted light. The 
idea involved is that light is reflected diffusely 
from the reflecting surface and that therefore 
the amount of light received back by the unit 


the 10-kc output of the phototube is a measure 
of the distance to the reflecting surface. 

It was originally hoped that the amplifier for 
the EMU could be used with the ODG, but it 
was later found that this might not always be 
possible. During tests transient signals are fre¬ 
quently generated in the ODG by light flashes 
of strobotrons. Each flash interferes with about 
three or four cycles of carrier frequency be¬ 
cause of poor low-frequency response of the 
EMU amplifier. The representatives of the Navy 
were aware of the limitations of the EMU ampli¬ 
fier when used with the ODG, but they did not 
request NDRC to design a different amplifier. 
The development of a suitable amplifier was left 
to naval personnel. 

The arrangement of the LU is shown in Fig¬ 
ure 5; and the circuit diagrams for the PU and 
LU are shown in Figure 6. The light source is 
an R-1130 facsimile lamp, the current of which 



PHOTO 

CELL 


RECEIVING 
LEN S 


CLAMPING 

SCREWS 


REFLECTING 
MIRR OR 


LENS 


LIGHT 
SOUR CE 


Figure 5. Top view of light unit. 


through a fixed aperature varies with the dis¬ 
tance of the reflecting surface from the aperture. 

The light source in the LU is modulated at 
10 kc. The received reflected light is focused 
upon a phototube; and the relative amplitude of 


is modulated at 10 kc. The light falls upon the 
transmitting lens, which is mounted in the end 
of the transmitting barrel. The latter extends 
from the source compartment into the lens-mir¬ 
ror compartment, and terminates in a small mir- 































160 


DEFLECTION-TIME MEASURING DEVICES 


ror set at an angle of 45 degrees with the axis of 
the barrel. The arrangement is such that the 
light collected by the transmitting lens is ren¬ 
dered very nearly parallel and is conducted along 
the length of the barrel, at the end of which it 
is reflected out of the LU by the mirror. The ex- 


reflecting material is attached to the point of 
maximum deflection for a test. The reflecting 
material is a piece of glass-beaded movie screen 
because its reflection is relatively insensitive to 
the angle of incidence. (The percentage of re¬ 
flected light from this material was constant 


POWER UNIT 



I 
I 

J 

Figure 6. Circuit diagram for power unit and light unit. 



tension of the transmitting barrel into the lens- 
mirror compartment is such that the light leaves 
the LU along the axis of the receiving lens, which 
is mounted directly behind the barrel in the wall 
between the lens-mirror compartment and the 
photoelectric cell compartment. The returning 
light, reflected from a distant object, is collected 
by the receiving lens and is focused upon the 
931-A photoelectric cell. 

Power is furnished to the LU by the PU. Con¬ 
nections between the two are made by means of 
a 5-conductor cable not over 20 ft in length. 
Terminals numbered 1 through 5 in the PU are 
connected respectively to terminals numbered 
1 through 5 in the LU. Terminals 6 and 7 of the 
PU are for use in adjustment of the percentage 
of modulation of the light source. 

As indicated in Figure 5, a 2- or 3-in. circle of 


for angles of incidence up to 30 degrees from 
the normal, and fell off slightly over 4 per cent 
at 45 degrees.) 

The LU must be fastened to a rigid support 
with an anti-shock mounting; it must be ar¬ 
ranged so that the light beam from the unit will 
strike the center of the screen, and so that the 
direction of travel of the screen during the 
course of the explosion will be along the axis of 
the light beam. It should be mounted as close 
to the test bulkhead as possible, allowing a 
reasonable margin of safety beyond the ex¬ 
pected deflection distance of the test. 

The PU may be placed in a convenient loca¬ 
tion not to exceed about 20 ft from the LU. The 
voltage regulator must be connected in the line 
at the PU so as to control only the voltage sup¬ 
plied to this unit. The output of the LU is at 






















































































































HISTORY 


161 


50-ohms impedance and may be cabled over rel¬ 
atively large distances, which permits the re¬ 
cording amplifier, oscilloscope, and camera to be 
set up at any convenient point. 

The calibration of the LU is dependent only 
upon the constants of the optical system; i.e., the 
apertures and focal distances involved. As a re¬ 
sult, the relative output of the unit for various 
distances should remain constant as long as 
these factors are not disturbed. Figure 7 shows 



Figure 7. Effect of focusing distance on relative 
response. 


the relative output for the system when focused 
for the screen material at different initial dis¬ 
tances. In obtaining each of these curves, the 
optical adjustments were made by setting the 
reflecting screen at a given distance and fo¬ 


cusing the optical system for maximum output 
at that screen distance. 

During calibration and during the time an 
actual test is in progress, stray light falling upon 
the screen material must be kept to a minimum, 
as the ultrasensitive 931-A phototube is easily 
overloaded. The presence of any appreciable 
stray light will render the calibration and test 
worthless. 

75 HISTORY 

When this project was first set up, a number 
of possible ways of measuring the bulkhead de¬ 
flections was considered. 1 Of those which were 
not subject to serious difficulties, the electro¬ 
magnetic method seemed the most direct. The 
development of the EMU was therefore under¬ 
taken. The final model was the result of a num¬ 
ber of experimental models used in actual tests 
and described in the original report. 1 

When the development of a less bulky unit for 
small-scale tests was undertaken, quite a num¬ 
ber of possible types were considered; 2 and it 
was decided that the ODG was the most promis¬ 
ing. The major problems in connection with 
ODG were: (1) the development of a projection 
system providing a nearly parallel beam of light 
from 1 to 2 in. in diameter; (2) the development 
of a method of modulating the light source at 
a high audio frequency; and (3) the develop¬ 
ment of a photocell-pickup amplifier. These prob¬ 
lems were susceptible to fairly straightforward 
solution; and the final unit resulted after some 
experimentation with an intermediate model. 








Chapter 8 


MEASUREMENT OF WALL THICKNESSES OF HOLLOW STEEL 

PROPELLER BLADES" 

By F. L. Yost » 


MILITARY REQUIREMENTS 

he National Defense Research Commit¬ 
tee was asked to develop a nondestructive 
method of measuring the wall thicknesses of hol¬ 
low steel propeller blades. One of the reasons 
for this request was that it was desired to test 
a large stock of blades which were suspected of 
having weakened areas because of grinding. 

The making of such measurements presents a 
number of problems. The size and shape of the 
blades introduce difficulty. In order to be of 
value the readings must be accurate to ±3 mils 
for a dimension which may vary from 40 to 300 
mils. And, finally, the method devised must be 
simple and rapid. The requirement was, there¬ 
fore, to develop a nondestructive, simple, rapid, 
easily managed method for measuring the wall 
thicknesses of propeller blades at various points 
to within an accuracy of 3 mils. 

82 SUMMARY OF DEVELOPMENT 

An instrument known as a penetron had been 
developed by the Texas Company for measuring 
the wall thickness of pipe. The principle involved 
was that a gamma-ray source and a gamma-ray 
counter was so situated outside the pipe that the 
wall of the pipe reflected and scattered to the 
counter gamma rays from the source. It was 
first hoped that this method, with minor modifi¬ 
cations, would meet the requirement. It soon 
appeared, however, that reflection from the sec¬ 
ond wall of the propeller would introduce diffi¬ 
culties. 

It was then decided to place the source inside 
the blade and to measure gamma-ray transmis¬ 
sion to the counter outside the blade. This tech¬ 
nique presented two major problems: (1) hold¬ 
ing the source exactly opposite to and on the 
axis of the counter for all points which might 

* AC-79. 

b Technical Aide, Division 17, NDRC. 


be measured, and (2) selecting a radioactive 
source which would give the required accuracy 
of measurement. 

The Texas Company solved the first problem 
by the construction of a pantograph which con¬ 
trolled the positions and attitudes of the source 
and counter. At the suggestion of Section 17.1, 
personnel engaged in nuclear research at MIT 
and at Ohio State College were called upon by 
The Texas Company for advice and assistance. 



Figure 1. General arrangement of apparatus for wall- 
thickness determination. 


In the measuring device as finally developed, 
a concentrated source of radioactive selenium 
(obtained by bombarding metallic arsenic with 
deuterons in a cyclotron) is pressed against the 
inner surface of the propeller wall. By means of 
the pantograph a Geiger-Mueller gamma-ray 
counter is pressed against the outer surface of 
the wall, directly opposite the source. The wall 
thickness between them can be determined from 
the length of time required to count a specified 
number of gamma rays. 

The equipment was installed at the Propeller 
Division of the Engineering Laboratory, of 



162 










DESCRIPTION AND TECHNICAL INFORMATION 


163 




-1 INCH- 

Figure 2. Radioactive-source holder. 


Wright Field, Dayton, Ohio. Extensive tests as 
to its stability and reproducibility proved the 
instrument to be satisfactory. As a final test 
approximately 100 locations were measured on 
a standard propeller blade. Holes were then 
drilled at the stations measured and the wall 
thicknesses were checked with an Ames mechan¬ 
ical indicator. It was assumed that the indicator 
was correct, although experience showed that 
measurements at different points on the periph¬ 
ery of the same hole varied up to 10 mils. The 
gamma-ray instrument yielded values which 
were well within the specified accuracy require¬ 
ments of ±3 mils. (Some of the measurements 
did have a much larger error, but they were all 
close to the shank at points where the panto¬ 
graph had to be tilted appreciably. At such loca¬ 
tions the source is shifted sufficiently from true 
alignment on the axis of the detector that errors 
are to be expected.) In general, the device com¬ 
pletely satisfies the requirements. 

In these tests it was found desirable to have 


the instrument warm up overnight. It required 
about 8 hours to measure 100 stations, or about 
5 minutes per station. Two to three minutes were 
required for two readings at each station (actual 
reading time is about one minute) and the rest 
of the time at a station was required for adjust¬ 
ment of the blade and pantograph. It was esti¬ 
mated that operation time could possibly be cut 
50 per cent by improvements in setting-up tech¬ 
nique and by the use of a stronger source. 

83 DESCRIPTION AND TECHNICAL 
INFORMATION 

The general arrangement of the instrument is 
shown in Figure 1. There is a holder which sup¬ 
ports the blade at its shank in a horizontal posi¬ 
tion and permits rotation of the blade about its 
longitudinal axis. Attached to the same frame¬ 
work is a two-arm pantograph. One arm sup¬ 
ports the radioactive source on the inside of the 
blade, the other, the detector on the outside of the 
blade. The pantograph can be moved along the 
length of the blade and can also be rotated in a 
horizontal plane. Such motion supplemented by 
rotation of the blade permits reaching every 
point inside the blade and keeping the detector 
axis perpendicular to the wall. 

During its passage through the wall from 
source to detector, the gamma-ray beam experi¬ 
ences a reduction in intensity due to absorption. 
The intensity of the beam at the detector there¬ 
fore decreases with increasing wall thickness. 
It was decided to have the source and detector 
pressed against the wall surfaces because in 
such an arrangement an increase in wall thick¬ 
ness results in a reduction of detected intensity 
for two reasons: (1) the absorption is greater; 
(2) the detector is farther from the source, thus 
reducing the solid angle subtended at the source 
by the orifice of the detector. Furthermore, it was 
much easier to separate the source and detector 



Figure 3. Detector and preamplifier circuit. 


rmn ratiiiiMi 


(aa& 







































r 6AF6G 


164 


WALL THICKNESSES OF HOLLOW STEEL PROPELLER BLADES 




Figure 4. Amplifier and scale-of-256 counter circuit. 

















































































































































































































































































































DESCRIPTION AND TECHNICAL INFORMATION 


165 


by the thickness of the wall than to hold them 
at a constant separation. 

It is desirable to have as large a change of 
intensity as possible result from insertion of a 
given thickness of steel between the source and 
the detector. The lower the energy of the gamma 
rays, the higher is the absorption coefficient and 
hence, the greater the change. However, most 
substances with low-energy gamma rays have 
rather short lifetimes and are accordingly un¬ 
suitable. Selenium, with a half-life of approxi¬ 
mately 180 days, seemed to be a suitable source 
material. 

The instrument was finally equipped with 
such a source in the form of HoSeO^ dissolved 
in nitric acid. The solution was evaporated to a 
dry powder and placed in a source holder shown 
in Figure 2. The holder was made of “Mallory 
1000,” an alloy containing over 99 per cent 
tungsten. The walls were made sufficiently thick 
so that gamma rays not emitted toward the de¬ 
tector are absorbed to a high degree (in order to 
reduce the scattering effect of neighboring parts 
of the wall and of the back wall of the propeller). 
The radioactive powder is placed in the opening 
which is then filled with molten paraffin, which 
prevents any dislocation, and a thin aluminum 
cover is added. The source is supported by a 
thin sheet of stainless steel which is sufficiently 
flexible so that the source secures a positive con¬ 
tact between it and the inside of the blade. 

The detector is a Geiger-Mueller counter tube 
of conventional design. Its current pulses are in¬ 
creased by a preamplifier, the electric circuit of 
which is shown in Figure 3. It involves a Neher 
quench circuit and two stages of pulse amplifi¬ 
cation. The detector and preamplifier are mount¬ 
ed in a shielded container which is connected by 
a multi-conductor cable to the rest of the circuit 
and the power-supply unit. The latter connects 
through a commercial voltage regulator to a 
110-volt a-c line and supplies the necessary volt¬ 
age to the preamplifier. 

Figure 4 shows a circuit in which pulses from 
the preamplifier are further amplified, equalized, 
and fed into a counter circuit with a scale of 
256 pulse.s per division. This arrangement allows 
the counting of every pulse, but the indicator 
need not operate so fast. Pulses from this 
counter circuit are fed into an electromechanical 


counter, the circuit of which is shown in Figure 
5. The mechanical counter makes one revolu¬ 
tion for 100 impulses. A contact is made by 
which an automatic electric timer is started at 
the zero position of the counter; and it is stopped 
after the counter has made three full revolu¬ 
tions. The timer therefore gives the time for 
counting 300 X 256 or 76,800 pulses. 

Due to the nature of radioactive decay any 
measurement of intensities has a certain statis¬ 
tical error. This error decreases with an in¬ 
crease in the number of observed impulses. For 
the figure given above the average statistical 
error is 0.36 per cent. 

Originally the Geiger-Mueller counter was not 
shielded against extraneous gamma rays, being 
open in all directions. It was soon found that 
with such an arrangement neighboring parts of 
the wall and the back side of the propeller blade 
influenced the reading. A lead plate of ^-in. 
thickness was, therefore, placed inside the alu¬ 
minum housing between its end plate and that 
of the counter. This plate has a hole at its center 


ELECTRO-MECHANICAL COUNTER +150 VOLTS CLOCK 



11-1. Sigma relay micro switch, 2,000 ohms resistance; 
closes at I 5 ma. 

R-2. Leach relay, 10,000 ohms resistance, Type 1054, 
coil No. 361; closes at 80 volts on coil. 

Electromechanical counter. Manufactured by Cyclotron 

Specialties Company: 

Two indicators: (1) 0-10 minutes for one revo¬ 
lution. 

(2) 0-0.10 minutes for one revo¬ 
lution. 

Operation on 110 volts 60 cycles per second; electro¬ 
magnetic controlled clutch starts and stops indi¬ 
cators; motor runs continuously. 

Accuracy of system. ± 0.01 second for starting and 
stopping operation. 

Operation: (1) With DPST switch in closed position, 
counter contact closed “locks” R-2, closing circuit 
to timing clock, starts clock. (2) With DPST 
switch in “open” position, counter contact closed 
“locks out” R-2, opening circuit to timing clock, 
stops clock. 































166 


WALL THICKNESSES OF HOLLOW STEEL PROPELLER BLADES 











/ 
































O 0.05 0.10 0.15 0.20 025 


Fe IN INCHES 

Figure 6. Calibration curve for shield with 24-inch hole. 

that is coaxial with the counter. After holes of 
various sizes had been tested, a diameter of % in. 
was chosen. Figure 6 shows the calibration curve 
for this hole. The thickness of the sheet inter¬ 


posed between source and detector is plotted as 
abscissa and the time in minutes to count 76,800 
pulses is plotted as ordinate. It was experimen¬ 
tally determined that the %-in. hole averaged 
the blade thickness over an area of 0.4 sq in. 

One disadvantage of the selenium source is 
that after a period of 180 days the intensity of 
the gamma-ray beam has decreased to half of 
its original value. For accurate measurements 
the decrease in primary intensity must be taken 
into account. Practically, it turns out that the 
decrease per 24 hours corresponds to the absorp¬ 
tion which is caused by a 2-mil thickness of steel. 
If a calibration curve as shown in Figure 6 is 
used, each day an additional 2 mils must be sub¬ 
tracted from the measured thickness. If the 
age of the source is unknown, a new calibration 
curve can easily be determined. 















Chapter 9 


DEVELOPMENT AND APPLICATIONS OF ELECTRONIC COUNTER CIRCUITS 

Bv George E. Beggs, Jr. :l and F. L. Yost h 


91 INTRODUCTION 

A n electronic counter is an instrument 
_ which measures and records the number of 
electric pulses it receives from a suitably de¬ 
signed network. The term “electronic counter” 
is here used to refer to the complete apparatus 
used for this purpose. Because of the low inertia 
of electronic systems, such a counter is capable 
of a counting speed thousands of times greater 
than that of the best mechanical counter. A scale- 
of-10 counter registers every tenth impulse; a 
scale-of-100 counter (e.g., two scales-of-10 in 
cascade) registers every hundredth impulse, and 
so on. Impulses entering the counter are divided 
again and again through successive units in 
cascade until the effective number is small 
enough to be recorded by a mechanical counter, 
the reading of which is then multiplied by the 
scale of the electronic counter. 

A decimal-system counter consists of a num¬ 
ber of elements—units order, tens order, hun¬ 
dreds order, etc.—which tell how many units, 
tens, hundreds, etc., have been counted. Any or¬ 
der consists of a series or ring of tubes which 
are rendered conducting (and later, nonconduct¬ 
ing) sequentially and repeatedly as successive 
impulses are fed to the order. One of the tubes 
in a ring is arranged not only to become conduct¬ 
ing in its proper sequence, but also to pass an 
impulse on to the next higher order, so as to 
record the number of times the complete ring 
has been traversed. At the end of a counting 
period, the count received by the order is de¬ 
termined by the number of complete traversals 
(indicated by the next higher order) and the 
position of the final conducting tube in the ring. 

For any order in a counter, the constants of 
the tubes, the circuit constants, and the type of 
circuit limit the speed with which one tube can 
be rendered conducting and the preceding one 
nonconducting. For any complete counter, the 
limiting factor in the counting is the speed of 

a Technical Aide, Sections 17.1 and 17.2, NDRC. 
b Technical Aide, Division 17, NDRC. 


the units-order counter which must correctly 
respond to all impulses; the restrictions on speed 
and performance of subsequent orders in a 
cascade are much less stringent. This project 
was concerned with increasing counting speeds 
of electronic counters. Accordingly, it was con¬ 
cerned with development of circuits for use as 
low orders in a complete counter, which would 
minimize the time involved in changing the con¬ 
ducting states of two successive tubes. 

92 MILITARY REQUIREMENTS 

When this work was undertaken electronic 
counters were available which would count ac¬ 
curately as many as 20,000 regularly spaced 
electric pulses per second using the decimal sys¬ 
tem, or 220,000 pulses per second using a scale- 
of-2 counter. There was no definite military re¬ 
quirement for development of counters with 
higher speeds, but it was believed that the de¬ 
velopment of reliable circuits capable of higher 
counting rates would be a useful undertaking 
and that valuable applications might result. For 
example, high speeds would make it possible to 
divide a time scale into very small units, thus 
permitting many types of precise measurements 
otherwise impossible. 

The simplest electronic counter is a scale-of-2 
type consisting essentially of two tubes which 
are made alternately conducting by successive 
impulses. It transmits a single impulse for every 
two it receives. If n such counters are arranged 
in cascade they are referred to as a binary count¬ 
er with a scale of 2". Such a counter is speedy, 
but, since it reports in powers of two, its read¬ 
ings are not convenient for interpretation. A 
more simple but less speedy arrangement is a 
decimal system reporting in powers of ten. Ac¬ 
cordingly, there was placed on the research the 
additional requirement that the system de¬ 
veloped be a decimal one. The initial objective 
in the work aimed at a reliable counting speed 
of 500,000 impulses per second (ips). 


167 




168 


DEVELOPMENT AND APPLICATIONS OF ELECTRONIC COUNTER CIRCUITS 


93 SUMMARY OF DEVELOPMENT 

Originally, contracts were entered into with 
the Massachusetts Institute of Technology, 18 '- 2 
the University of Chicago, 14 - 17 and the National 
Cash Register Company 1 ' 7 for the development 
of ultra-high-speed electronic counter circuits. 
Each contract required the submission of one 
or more sample counters. Under the three con¬ 
tracts, various circuits were considered and 
tested sufficiently to estimate their possibilities. 
These are discussed in Section 9.5 under History. 
Ultimately the first two contracts were ter¬ 
minated and work was concentrated under a 
contract with the National Cash Register Com¬ 
pany. Resulting from these original contracts, 
a new contract 813 was written with the National 
Cash Register Company, under which applica¬ 
tions of the counter circuits were to be developed. 

The final decimal counter developed under this 
project has six orders, so that a total accumula¬ 
tion of 999,999 is possible. The units order is a 
scale-of-16 binary counter which is reset to zero 
at every tenth impulse. This counter has a speed 
of 4,000,000 ips when operating as a true scale- 
of-16 counter and of 1,500,000 ips when operat¬ 
ing as a scale-of-10 counter. All but the units 
order are gas-tube counters with lower speeds. 
Including all orders, the counter has a reliable 
counting speed of 1,000,000 ips, or possibly 
slightly higher. 

In order to test the accuracy of this counter, 
an instrument was developed to generate sine- 
wave trains of a predetermined number of cy¬ 
cles. By use of it there may be applied to the 
counter by a 1-mc signal a known number of 
pulses varying from 1 to 999,999. A high-fre¬ 
quency oscillator is used with a “gate” tube, 
which effects the initiation and termination of 
the megacycle signal on the transmission line. 
The gate tube is controlled by a “start” and a 
“stop” gas triode, which is activated by a local 
counter when the predetermined number of im¬ 
pulses is reached. 

Since electronic counters provide a means of 
determining the number of discrete, rapidly re¬ 
curring electric impulses, they can be used as 
translation devices in an intelligence transmis¬ 
sion system, the intelligence being conveyed by 
the number of impulses comprising a group. A 
system was devised for transmitting by radio 


to a receiver-counter any number from 1 to 
99,999. The maximum time the transmitter is 
on the air broadcasting any number is 0.0017 
second, or less, so that on ordinary communica¬ 
tion receivers the only evidence of transmission 
of a five-character group is a relatively weak 
“click.” In the receiver-counter there are stor¬ 
age tubes for recording any five 5-digit numbers 
until read-out is desired. These tubes will “re¬ 
member” a number indefinitely, until they are 
cleared by the operator of the apparatus. Fur¬ 
thermore, the system will refuse to receive 
numbers when there are no tubes available for 
storage. 

In connection with the development of the 
high-speed counter, an electronic gate circuit 
was developed to make possible the measure¬ 
ment of the interval between two electrical im¬ 
pulses. The two impulses open and close a gate 
feeding megacycle pulses into the counter. The 
precision in determining the interval between 
the two impulses depends on that of the fre¬ 
quency being fed to the counter with the possi¬ 
ble error of ±1 microsecond introduced in the 
gate circuit itself. 

The original system was delivered as an exper¬ 
imental unit to determine its usefulness to the 
Aberdeen Proving Ground where the Ordnance 
Department found that its accuracy and adapta¬ 
bility warranted the procurement of additional 
models because the counter and associated gate 
circuit mentioned proved useful in the measure¬ 
ment of muzzle velocities of various sizes of 
shells. 

The communication system was demonstrated 
to numerous Service personnel in late 1942 and 
early 1943 but the consensus was that there was 
no requirement for the apparatus. 

In connection with this work, circuit and 
theoretical data were furnished to various 
NDRC groups as well as for Service uses. 


94 DESCRIPTION AND TECHNICAL 
INFORMATION 

911 Ultra-High-Speed Decimal Accumulator 7 

The wiring diagram of the complete high¬ 
speed decimal accumulator [HSDA] is given in 
the contractor’s report. 78 The instrument con- 



DESCRIPTION AND TECHNICAL INFORMATION 


169 


sists essentially of an assembly of five sections: 
the power and control, the indicator, the units- 
order high-speed resetting binary counter, the 
units-order decoder, and the gas-tube counter. 

The units-order counter, which makes the 
high speed possible, resulted from development 
work on a high-speed resetting binary counter 
[RBC] originally suggested under the contract 


livers negative impulses to that stage which 
converts the sine waves into the required step 
impulses. 

The first scale-of-2 stage of the RBC operates 
on every input impulse; the second, on every 
second impulse; the third, on every fourth im¬ 
pulse ; and the fourth on every eighth impulse. 
Each stage is in its zero position when its A tube 



with the University of Chicago. 14 Figure 1 
shows the elements of the final design of the RBC 
circuit. 713 (It was modified somewhat on incor¬ 
poration in the HSDA, but this figure serves to 
indicate the method of operation.) Basically the 
RBC is a scale-of-16 counter (i.e., four scales-of- 
2 in cascade) with interstage discriminators and 
recycling system which resets the counter to 
zero at the tenth count. A transfer impulse to 
the next higher denominational order is effected 
every tenth count (i.e., every time the counter 
resets or recycles). A two-tube impulse shaper 
precedes the first stage of the counter and de¬ 


is conducting. After a count of eight, the B tube 
of stage four will be conducting; and after nine, 
the B tubes of the fourth and first stages. The 
tenth count triggers the first stage to its zero 
position, initiating a signal to the second stage 
which triggers from A to B. 

The RBC now resets; that is, the second and 
fourth stages trigger back to the A position, 
stages 1 and 3 being already there. Resetting is 
accomplished by applying a negative impulse 
to the suppressor grids of tubes 2B and 4B 
(which are conducting at a count of 10) from 
the resetting tube V7, which conducts each time 


mzmmmmAk 
































































































































































































170 


DEVELOPMENT AND APPLICATIONS OF ELECTRONIC COUNTER CIRCUITS 


4B and 2B are conducting. The negative impulse 
renders tubes 2B and 4B nonconducting, which 
makes 2A and 4A conducting and thus effects 
the resetting. 

The units counter of the HSDA has a maxi¬ 
mum counting speed of 4,000,000 ips when op¬ 
erating as a true scale-of-16 binary counter. 
This high speed is obtained by reducing tube 
and distributed capacities to a minimum, and 
selecting a tube having a very low plate resis¬ 
tance. The counter has a maximum speed of 
1,500,000 ips when operating as a scale-of-10 
counter. The resetting system limits the scale- 
of-10 speed. 

The complete HSDA has six orders, so that a 
total accumulation of 999,999 impulses is pos¬ 
sible. Suitable arrangements are made for trans¬ 
fers of impulses from each decade to the one of 
higher order. All but the units order are gas-tube 
counters. The circuit shown in Figure 9 is essen¬ 
tially that used for the tens-order counter, which 
operates at 100,000 ips when the input signal is 
1,000,000 ips. (There was some modification of 
this circuit in connection with its incorporation 
in the HSDA; e.g., addition of another tube for 
transferring impulses to the hundreds order, 
etc.) The hundreds-, thousands-, ten thousands-, 
and hundred thousands-order gas-tube counters 
are lower-speed thyratron rings with a maxi¬ 
mum speed of 15,000 ips. These orders differ 
from the tens order only in circuit constants. At 
the termination of a counting period, the count 
which is retained is indicated by an electro¬ 
mechanical indicator, which senses the counters 
and indicates the ignited tubes. The units-order 
counter, however, has its count retained in the 
binary system of notation, and it is therefore 
necessary to decode or translate the binary no¬ 
tation to the decimal notation so that the deci¬ 
mal indicator can sense the resulting decimal 
values. 

Controlled Impulse Generator 7 ' 

A complete wiring diagram for the controlled 
impulse generator is given in the contractor’s 
report. 7 * 3 It generates groups of from 1 to 999,- 
999 impulses at the rate of 1,000,000 per second. 
It consists of a self-contained power supply, a 
relay-control system, an ultra-high-speed accu¬ 
mulator, a six-column keyboard, a high-fre¬ 


quency oscillator, and a start-stop system asso¬ 
ciated with a gate tube. 

The generator accumulators are counterparts 
of those used in the receiver, with one important 
exception: whereas the receiver-counter rings 
start on zero and advance to some other position, 
the generator counters can be started anywhere, 
and always end on the same position, which is a 
specified capacity of the accumulator. When 
that position is reached a stop tube closes the 
gate tube, which removes the signal from the 
receiver counter. In operation, the setting up of 
a number on the keyboard of the generator con¬ 
ditions the tubes in the generator’s accumulator 
so that the accumulator merely needs to count 
that number to reach its specified capacity (i.e., 
setting up a number ignites tubes in the accumu¬ 
lator to indicate that the complement of that 
number has already been counted). 

The application of the indicated number of 
impulses to the receiver-counter is controlled by 
a gate tube, an 1852 used as a suppressor-modu¬ 
lated Class A amplifier, whose output is supplied 
to the local accumulator and also to the output 
terminal. The suppressor is modulated by a 
square wave furnished by two gas tubes known 
as start and stop respectively. When the start 
tube becomes conducting it makes the suppres¬ 
sor of the gate slightly positive and permits radio 
frequency to appear in the plate circuit. Firing 
of the start tube is synchronized with the r-f 
voltage on the grid of the 1852 so that only full 
cycles can be sent out, thus insuring that the 
first cycle will be counted. When the capacity of 
the accumulator is reached (i.e., when the num¬ 
ber of impulses set up in the keyboard has been 
transmitted) the stop tube biases the gate’s sup¬ 
pressor beyond cutoff and no further impulses 
are transmitted. 

94 3 Electronic Counter Communication 
Device 11 

The electronic counter communication device 
[ECD] consists of three units: a pulse-produc¬ 
ing device or modulator (for wiring diagram 
see Figure 2 of the contractor’s report 11 ), a 
radio transmitter (Figure 4 of the contractor’s 
report 11 ), and a wide-band radio receiver with 
associated counter circuits and read-out devices 
(Figures 7, 11, 12, 13, and 14 of the contractor’s 





DESCRIPTION AND TECHNICAL INFORMATION 


171 


report 11 ). The arrangements of the impulser, of 
the impulse counter, and of the storage tubes and 
indicator are shown in Figures 2, 3, and 4 below. 

The modulator has a keyboard (Figure 2) sim¬ 
ilar to that found on computing machines, con¬ 
sisting of five columns of keys numbered 1 to 9. 
Each key has a small thyratron tube associated 
with it and each column of keys has also two 
spare thyratrons, one for the digit 0 (for which 
there is no key) and one to transfer the electric 
indications from one decade to the next. The 



Figure 2. View of impulser and keyboard. 


modulator, therefore, has a total of 55 small 
thyratrons, plus associated power-supply cir¬ 
cuits, amplifier circuits, and pulse-sharpening 
circuits. 

Any desired 5-digit number is set up on the 
keyboard and is sent by depressing a “send” 
key. The depression of that key initiates a train 
of impulses produced by the miniature thyra¬ 
trons and their associated RC coupling circuits. 
These pulses occur at a rate of 40,000 per second, 
with time spacing between the indications of 
various decade banks to allow transfer of cir¬ 
cuits from one bank to the next. The total trans¬ 
mission time required for any 5-digit number is 
0.0017 second, or less. This is possible because 
a decimal system is used. A number such as 
99,999 does not require the generation of 99,999 
pulses; it requires only 55 pulses, that is, nine 
pulses for each decade, plus an extra pulse in 
each decade to represent the zero key, plus a 
transfer pulse for each decade. 



Figure 3. Impulse counter. 


The pulses so produced are applied to an r-f 
transmitter operating at 7 me. After being 
transmitted by radio carrier, the signals are re¬ 
ceived in a wide-band superheterodyne receiver, 
and are then applied to appropriate counting cir¬ 
cuits (Figure 3). These counting circuits are 
practically identical to the pulse-producing cir¬ 
cuits in the modulator. The count made by these 
circuits in any decade is applied electrically to 
a multi-section storage tube, which remembers 
the number of counts received. This “memory” 
is accomplished by tripping one of the thyratron 
sections of the storage tube in accordance with 
the count received. This section of the storage 



Figure 4. Electronic storage chassis and mechanical 
read-out dial. 






172 


DEVELOPMENT AND APPLICATIONS OF ELECTRONIC COUNTER CIRCUITS 


tube remains conducting until the plate voltage 
is removed by opening a switch manually after 
the read-out. 

The final indication of the number received, 
counted, and remembered by the apparatus is 
obtained on a mechanical read-out dial (Figure 


For coded communications it would be pref¬ 
erable to have a system in which numbers from 
0 to 25 could be counted and stored. Such a sys¬ 
tem could be built up. A system built on a base 
of 9 rather than 25 was developed because stor¬ 
age tubes with 10 sections were readily available. 




4) which indicates the sections of the various 
storage tubes conducting at the time the num¬ 
ber is read. There are sufficient storage tubes in¬ 
corporated in the apparatus to allow the indica¬ 
tion of any five 5-digit numbers. This means that 
25 numerals from 0 to 9 can be remembered 
simultaneously and read out at will on the me¬ 
chanical system. The storage tube will remember 
a number indefinitely until it is cleared by the 
operator. Furthermore, the system will not re¬ 
ceive numbers when tubes are not available for 
storage. 


Electronic Gate Circuit 12 

The wiring diagram of the high-speed counter 
auxiliary one-megacycle electronic gate [EG], 
for measuring the interval between two electric 
impulses, is shown in Figure 5. Essentially, the 
EG is an 1852/6AC7 amplifier stage through 
which the megacycle frequency is fed to the 
counter unit. The 1852 is arranged for suppres¬ 
sor modulation by a square wave which is gen¬ 
erated by two miniature thyratrons controlled 
by the interval-marking pulses. The suppressor 





























































































HISTORY 


173 


of the EG is normally held sufficiently negative 
to prevent conduction. Upon receiving the start 
signal, the suppressor is swung slightly positive 
and the stage operates as a normal Class C ampli¬ 
fier. Application of the stop signal causes the 
suppressor to be returned to its cutoff potential 
and the 1-mc output of the stage disappears. The 
number of cycles passed during the interval is 
counted by the high-speed counter and serves to 
determine the time interval. The accuracy is 
±1 microsecond. 


beam from the gun moves along the axis of the 
tube and is deflected by B and D, so that the 
electrons must move on the surface of a cone. 
In addition, the B segments have the proper 
electric potentials to keep the electrons in a pen¬ 
cil beam and directed in such a way as to strike 
the region which is clockwise from the support¬ 
ing lead of one of the collecting grids C. Once 
the electron beam is directed on a certain C ele¬ 
ment, it continues to strike at that portion of 
the element as long as all potentials remain un- 





OF BRASS 


9.5 HISTORY 

In work leading up to the final circuits, a 
number of ideas and circuits were considered 
and tested, and it is felt that they merit some 
discussion. 

Electron Ratchet Tube 14 * 

One of these ideas was the electron ratchet tube 
[ERT], as illustrated in Figure 6. This was sug¬ 
gested by the University of Chicago. An electron 


changed. If a positive pulse is applied to the 
central cone, the electron beam will be pulled 
in along a radius and will strike the C element 
which is next in a position clockwise to the one 
from which it departed. When the beam strikes 
the new C element, potentials adjust themselves 
automatically to move it clockwise on the ele¬ 
ment to a position where a positive impulse on 
the cone will move it in to strike the next C 
element. Each positive impulse moves the beam 


O 




































174 


DEVELOPMENT AND APPLICATIONS OF ELECTRONIC COUNTER CIRCUITS 



Figure 7. Conjugate-pair counter. 


forward one element. The tube could therefore 
be used for a counter, with one C element ar¬ 
ranged to transmit an impulse to another decade 
each time the beam struck it. By a slight altera¬ 
tion of the shapes of the C elements arrange¬ 
ments could be made for positive pulses to cause 
clockwise rotation, and negative pulses to cause 
counterclockwise rotation. A model tube of the 
clockwise type was constructed but time was not 
available for testing it as a high-speed counter. 

Conjugate-Pair Counter Circuit 7 * 
Figure 7 is the circuit diagram of a scale-of-10 
counter, a configuration of ten high-vacuum 
tubes arranged in five interconnected trigger 
pairs. Five of the ten tubes are always conduct¬ 
ing. In the early research the circuit showed 
promise of responding to high impulsing rates, 
but other types of counter circuits later super¬ 
seded it. The highest counting speed observed 
during the tests of this circuit was 300,000. It 
was felt that with further development a special 
vacuum tube might enable the circuit to operate 
at 1 me. 

Dekatron Counter Circuit 71 
This circuit, shown in Figure 8, was one of the 
early ones considered. It is a seven-tube scale- 


of-10 counter — an arrangement of a scale-of-2 
counter cascaded with a scale-of-5 counter. The 
scale-of-5 section is the novel part of the cir¬ 
cuit. One tube of the five is held in a conducting 
state by the joint action of the four nonconduct¬ 
ing tubes. In a sense, the circuit is a trigger 
circuit with one tube acting as the complement 
of the other four. The counter is read-out by an 
electromechanical indicator through a decoding 
network. This counter operates well at 25,000 
ips. 

9 5 4 High-Speed Tliyratron 

Counter Circuits 78 

Previous thyratron scale-of-10 circuits relied 
on grid priming and extinction by cathode coup¬ 
ling for operation, thus limiting the counting 
speed to that determined by the deionization time 
of the previous tube. By introducing a new 
method of anode extinction through the use of a 
single common anode resistance and individual 
cathode condensers, tube deionization does not 
have to be completed until it is again time for 
that tube to conduct, increasing the limiting 
speed by a factor equal to the number of tubes 
in the ring — in this case, ten. Further increases 
in speed are made possible by reducing the de- 


















































































































HISTORY 


175 


ionization time of the tube, which can be ac¬ 
complished by changing the gas pressure and 
electrode size and spacing. 

A scale-of-10 counter involving the above fea¬ 
tures was developed. It operated well at 100,000 
ips and would count at higher speeds if anode, 
grid, and heater potentials were carefully con¬ 
trolled. The counting speed of a gas-tube counter 
can be doubled by inserting a scale-of-2 counter 


as the 10 2 , 10 3 , 10 4 , and 10 5 denominational or¬ 
der counter in a multiple-decade counter. Ac¬ 
cordingly, a thyratron counter with a top speed 
of about 12,000 ips was designed. It is compara¬ 
tively simple to design such a counter with a 
tube grid bias during conduction of only —15 v. 
This counter has the same configuration as the 
higher speed gas-tube counters, but different cir¬ 
cuit values. Another gas-tube counter of medium 



ahead of the gas-tube counter. Thus it is possi¬ 
ble to count at 200,000 ips by using a scale-of-2 
high-vacuum counter and a scale-of-5 thyratron 
ring, the combination being a scale-of-10 count¬ 
er. To improve tube life this circuit was later 
changed so as to operate at the same speed with 
smaller grid potentials. The resultant circuit, 
which is the basis of the tens-order counter in 
the HSDA, is shown in Figure 9. 

9-5.5 Medium- and Low-Speed 

Thyratron Counters 711 

In many applications it is not necessary to pro¬ 
vide thyratron counters with speeds greater 
than 10,000 ips, such as when the ring is used 


speed was developed, operating at 50,000 ips 
with —20 v on the grid of a conducting tube. 
This counter differs from the high-speed ones 
only in circuit constants. All the counters use the 
miniature thyratron designed for high-speed 
operation. 

Binary to Decimal Converter 71 

This development consists of a binary counter 
of many stages wherein the complete count is 
received, and a translater or converter which 
impulses the binary count into a decimal accu¬ 
mulator or totalizer after the counting period 
has ended. Counting can be done with this circuit 
at 4 me. Approximately 1 second is required for 
conversion. 















































































































































176 


DEVELOPMENT AND APPLICATIONS OF ELECTRONIC COUNTER CIRCUITS 


PULSE LINE 



First Counter Model 7 * 

The wiring diagram of the first counter model 
developed by the National Cash Register Com¬ 
pany is given in the contractor’s original report. 4 

The units-order counter comprises a scale-of- 
2 followed by a scale-of-5 gas-tube counter. The 


tens- and hundreds-order decades are thyratron 
rings of ten tubes each. A motor-driven electro¬ 
mechanical indicator senses the counter decades 
and indicates the counts after the counting pe¬ 
riod. After each count the instrument can be 
reset to zero by operating the relay system which 
is a part of the instrument. 
































































































Chapter 10 


DEVELOPMENT OF METHODS FOR DETECTING DEFECTIVE 
ROTATING BANDS ON PROJECTILES a 

By Clark Goodman b 


INTRODUCTION 

HE OBJECT of this investigation was to de¬ 
velop a nondestructive production method 
for detecting improperly banded projectiles. The 
gilding-metal band serves as a seal to prevent 
the escape of the powder gases past the shell, 
and to impart rotation to the shell as it is forced 
through the rifling of the gun barrel. Improperly 
seated bands have a marked effect upon the muz¬ 
zle velocity of the shell, and this results in a wide 
dispersion in the range of the projectiles. 

102 BANDING 

The clearance or gap that exists between the 
band and its seat in improperly banded shells 
may take one of several forms. 

1. The band is pressed firmly against the 
knurling on the band seat but has not been forced 
down against the band seat between the knurls. 

2. The band is loose at all points. It not only 
does not make contact with the band seat, but 
also has sprung away from the knurling. 

3. The band is not coaxial with the shell body; 
that is, it may fit tightly at one edge of the band 
seat while clearance exists at the other edge. 

In general, the clearance, whatever its type, 
is not uniform about the circumference. Values 
of clearance amounting to as much as eight or 
ten mils and even more have been found in poorly 
banded shells. 

A uniform clearance of one mil around the 
circumference of a shell is equivalent to a reduc¬ 
tion in the outside-band diameter of two mils. 
Tests at proving grounds have shown that a va¬ 
riation of band diameter in properly banded 
shells also affects the muzzle velocity and range 
of the projectile. A shell having a tightly seated 
band, 4.218-in. diameter, would have a muzzle 
velocity practically identical with a shell having 

* OD-151. 

b Technical Aide, Division 17. 


a poorly seated band of 4.220-in. diameter and 
an average clearance around the circumference 
of 1 mil. The Ordnance Specifications permit a 
variation in band diameter of ±0.003 in. from 
a nominal outside diameter of 4.220 in. When a 
shell is fired which has either clearance under¬ 
neath a band of the correct diameter or has a 
tightly seated under-sized band, full powder 
pressure is not applied to the shell, and there is 
a clearly defined flash of burning powder at the 
muzzle and the velocity of the projectile is below 
normal. If the shell band is of the correct diam¬ 
eter and is properly banded, then the flash is 
not observed inasmuch as the powder is com¬ 
pletely consumed before the shell leaves the gun. 

The effect of band clearance on the muzzle 
velocity of a 105-mm shell amounts to a reduc¬ 
tion of 2.6 (Aberdeen Proving Ground) to 3.2 
(Southwestern Proving Ground) fps per mil of 
clearance at a muzzle velocity of 1,030 fps. 
Therefore, the effect of a variation of 1 mil 
in band diameter would produce approximately 
1.5 fps change in muzzle velocity. 

10 - 3 PROBABLE DISPERSION OF 

MUZZLE VELOCITY 

In addition to banding, other factors produce 
variations in muzzle velocity. The most impor¬ 
tant of these is the powder. The proving grounds 
report that powder irregularities account for a 
variation of ±7 in a muzzle velocity of 1,030 fps. 
It is evident, therefore, that as a result of varia¬ 
tions in banding and powder one may expect a 
dispersion in muzzle velocity of approximately 
±12 in a velocity of 1,030 fps in correctly banded 
shells whose band diameters conform to the al¬ 
lowable specification limits. Tests at the proving 
grounds show that more than 95 per cent of the 
good shells have a muzzle velocity within ±10 
fps of the reference shells. 

It should be observed, therefore, that the abil¬ 
ity to detect variations in muzzle velocity due 



177 



178 


METHODS FOR DETECTING DEFECTIVE ROTATING BANDS 


to poor band seating is dependent upon the pos¬ 
sibility of correcting for or taking into account 
the variations in velocity resulting from the 
foregoing causes. 

104 METHODS INVESTIGATED 

The following methods were considered but 
did not progress beyond the laboratory stage: 
air leakage test, 1 ball rebound test, la heat flow 
method, lb and supersonic tests I and II. lc A num¬ 
ber of other methods were tested and rejected 
as unsuitable for production tests. These include 
the Herzog test, 3 the inductance method, 1,1 an 
acoustical test, le - 3a current and potential contact 
tests, lf ’ 3b and thermal tests. 1g - 2>3c 

Herzog Test. This method was suggested by 
Gerhard Herzog of The Texas Company. In this 
method a %-in. hole is drilled through the band 
and a manometer containing air at a pressure 
of approximately 25 lb is connected to the hole. 
Then a cock is opened connecting the manometer 
to the space underneath the band and the fall 
in air pressure noted. It is assumed that the fall 
in pressure will be related to the volume of the 
air space underneath the band and, therefore, to 
the average band clearance. 

This method, however, is not satisfactory be¬ 
cause the clearance is rarely, if ever, uniform 
around the circumference, and the hole may be 
drilled at a point in the band where no gap 
exists. Gaps are seldom continuous and, even 
in tight bands, there is often some leakage of air 
at the edge of the band. Therefore, the test was 
not carried beyond the laboratory stage. 

The Inductance Method. In the inductance 
method, the change is measured in the indue- * 
tance of a coil placed around the band of a shell. 
This method was tried over at a wide range of 
frequencies in both the laboratory and at the 
Southwestern Proving Ground. As no correla¬ 
tion was found to exist between the muzzle ve¬ 
locities and the inductance values, the method 
was discarded. 

Acoustical Test. The acoustical method was 
described in the Progress Report of May 25, 
1944. The sound-testing equipment was taken to 
the Southwestern Proving Ground, where sev¬ 
eral hundred shells were given the sound test 
and fired to obtain their muzzle velocities. 

The muzzle-velocity data and the sound re¬ 


sults failed to show any evidence of correlation, 
and the method was discarded as unsuitable. 

Current and Potential Contact Test. This meth¬ 
od was developed by the Sperry Products, In¬ 
corporated, and the first Sperry unit was near¬ 
ing completion when the School of Engineering 
at Johns Hopkins University was brought into 
the picture. The U. S. Ordnance Department was 
so impressed by the initial results that three 
Sperry units with their auxiliary equipment 
were purchased under this contract and fur¬ 
nished, one to the Pittsburgh Ordnance District 
(Pullman Standard Car Company plant at But¬ 
ler, Pa.), one to the Aberdeen Proving Ground, 
and one to the Jefferson Proving Ground. The 
Aberdeen Sperry unit was sent abroad and used 
in England for a time. In addition, the U. S. 
Ordnance Department placed an order for a 
fourth unit for the Southwestern Proving 
Ground. 

The method was given a thorough field trial. 
The Jefferson Proving Ground carried out an 
exhaustive and careful study of its possibilities. 
The results proved that the method could not be 
relied upon to separate poorly banded from 
properly banded shells. It was, therefore, dis¬ 
carded as unsuitable and untrustworthy. 

Thermal Test. In this test, the band is coated 
with an opaque substance that melts at a low 
temperature. The shell is then placed in a sole¬ 
noid and subjected to an alternating magnetic 
field for a short period of time. The resulting 
current flow in the band heats it but, if the band 
is tightly seated, the heat flows rapidly into the 
steel body of the shell and a long time is required 
to melt the wax or substance used. If there is 
clearance under the band, then the air gap inter¬ 
feres with the flow of heat, and the wax is quick¬ 
ly melted over such areas. 

The method, because of its simplicity, speed 
of operation, and minimum of equipment re¬ 
quirements, was given an exhaustive study. It 
was not recommended, however, as a production 
test because on many tightly banded shells there 
is dirt, grease, oxides, or a combination of them 
on the band seat. This foreign material acts as a 
heat insulator and results in rejection of shells 
that fire with proper muzzle velocity. If it were 
feasible in manufacture to keep the band seats 
and the inside surfaces of the bands bright and 
clean, the method would be a most useful one. 



X-RAY METHODS 


179 


Although the adoption of the thermal test as 
a production test of banding is not recom¬ 
mended, nevertheless it is recommended for 
checking the uniformity of operation of banding 
presses and similar equipment. It provides a 
simple, inexpensive method of checking the op¬ 
eration of banding machines. 

Two other methods were investigated. The 
first of these involves the use of an X-ray beam 
to reveal any clearance under the band. How¬ 
ever, it was found that the firing data and the 
X-ray-measured clearances do not give good cor¬ 
relation. The method requires considerable ex¬ 
pensive equipment and a closely controlled volt¬ 
age supply. It, therefore, is not recommended 
as a production test for improperly banded 
shells. The method, however, furnishes a clear 
picture of any space that exists between the 
band and its seat, and it is easily applied to the 
bands of shells of any size. In addition, the ap¬ 
paratus is versatile and has many possible appli¬ 
cations. The installation of an X-ray Geiger- 
Mueller testing equipment at a research center 
such as the Aberdeen Proving Ground was rec¬ 
ommended. 

The best correlation with firing data was ob¬ 
tained with a compression test method (the 
Klipsch test) developed at the Southwestern 
Proving Ground. It was recommended for adop¬ 
tion as a production test method. In the Klipsch 
test two shoes, each of the same dimensions and 
spaced 180 degrees apart, were used. The pres¬ 
sure shoe covers an area of %x% in. and slightly 
less than 10 degrees on the band of a 105-mm 
shell. The total pressure applied is slightly in 
excess of 70,000 psi, and the resulting deforma¬ 
tion is measured either by a dial gauge in the 
testing machine or by a micrometer. 

Compression tests have one advantage that 
none of the other test methods possesses; name¬ 
ly, that they are independent of the presence 
of oxides and dirt between the band and its seat. 
If a band is properly seated, these foreign sub¬ 
stances are confined under considerable pressure 
and, therefore, do not affect the firing. 

10 s X-RAY METHODS 

The object of this test was to investigate the 
possibility of using the Geiger-Mueller counter 
in place of X-ray radiographs which had in¬ 
dicated the feasibility of using X-rays for the 


detection of improperly banded shells. Radio¬ 
graphs had been taken on a large number of 
shells at the Iowa Ordnance Plant, as well as 
elsewhere, and the shells had then been test- 
fired. Contrary to the tests at the Aberdeen 
Proving Ground, the muzzle velocity of these 
shells had correlated well with the average value 
of air gaps estimated from the radiographs 
taken at one to three positions on the shells. It 
was, therefore, thought that an X-ray test which 
could measure air gaps directly would give an 
indication of the expected muzzle velocity of the 
shell. 

Apparatus 

To adapt X-ray technique to the band-clear¬ 
ance problem and to provide a test that is as 
rapid, quantitative, and inexpensive as possible, 
a Geiger counter instead of a photographic film 
was used to determine the radiation passing 
through the shell. 

The X-ray machine finally used is a 220-kv d-c 
machine with the tube passing a current of 10 
ma. The tube voltage of 218 kv actually used is 
obtained from a step-up transformer whose pri¬ 
mary voltage is approximately 206 v. The pri¬ 
mary voltage must be kept constant during a 
test run, within a volt or two. For this reason, 
the a-c generator supplying the primary voltage 
is driven by a d-c motor run from a storage bat¬ 
tery. Some means of providing a constant volt¬ 
age to the X-ray circuit input is essential. 

The detailed arrangement of the apparatus 
was described in Progress Report OSRD 4576. 
A photograph of the final test set-up is shown 
in Figure 1. The shell is seen mounted just to 
the right of the X-ray tube housing, and still 
further to the right are the collimator and the 
Geiger-Mueller tube enclosed in its lead housing. 
Both a direct-reading and a recording milliam- 
meter were available for measuring the output 
from the counter circuit. This circuit and its 
associated apparatus are not shown in Figure 1, 
but the motor-drive mechanism for moving the 
shell across the X-ray beam is- apparent. 

10 - 5 - 2 Adjustments 

A shell, with clearance between its band and 
band seat, is placed in such a position that the 
X-ray beam passes through the band only. As the 
shell is moved further into the beam until the 





180 


METHODS FOR DETECTING DEFECTIVE ROTATING RANDS 



Figure 1 . Arrangement of apparatus for X-ray method. 


latter passes through both the band and the 
steel-shell body, the intensity of the X-ray beam 
reaching the Geiger tube will decrease to a mini¬ 
mum value, increase to a maximum, and then 
decrease again. The difference between the mini¬ 
mum and maximum current reading is called 
the spread and is an indication of the gap be¬ 
tween the band and the shell body. It is ob¬ 
viously desirable to test a shell as rapidly as 
possible without loss of accuracy in measuring 
this spread. This requires the determination of 
the best speed for driving the shell across the 
X-ray beam and the optimum value of capaci¬ 
tance for the integrating capacitor in the coun¬ 
ter circuit. 

The best combination of shell speed and ca¬ 
pacitance is that which gives maximum spread 
of the milliammeter reading in minimum time 
without objectionable fluctuation of the needle. 
For a given shell speed, if the capacitance is too 
high, the meter does not respond rapidly enough 
to indicate the complete spread while, if the 
capacitance is too low, the needle fluctuates to 
such an extent that the maximum and minimum 
points cannot be determined accurately. The 
optimum values were found to be a capacitance 


of 10 fj .f and a shell speed of approximately 0.025 
in. per minute. This permits a shell to be X-rayed 
at 36 points in approximately 45 minutes. 

The air gap between band and band seat may 
take any one of several different forms, some 
being fairly regular and others quite irregular. 
The problem is further complicated by the 
ridges in the band seat and the depressions in the 
band produced in the banding operation. At first, 
shells were X-rayed at only four points around 
the circumference. It was soon found that the 
average spread at these four points might be con¬ 
siderably different from the true average spread 
of the entire shell, since some very large gaps 
were found to extend over as little as 10 degrees 
of the circumference. Such irregular gaps ex¬ 
tending for only a few degrees should have little 
effect on the muzzle velocity of a shell, but they 
may have some effect, so it was decided to take 
readings at 10-degree intervals. 

In March 1945, 110 shells which had pre¬ 
viously been tested by the X-ray Geiger-counter 
test at the Johns Hopkins University were fired 
at the Aberdeen Proving Ground. The results of 
this firing test established that the X-ray test 
does not give an indication of the muzzle velocity 

















RESULTS 


181 


of a shell. It may be able to separate good lots 
from bad lots, but its results do not correlate 
with the firing data for shells of a single lot. 
This seems to indicate that the air gap is not the 
only factor in the variation of muzzle velocity in 
the shells tested. 

10 6 KLIPSCH COMPRESSION TEST 

The Klipsch compression test is based on the 
assumption that any air gap beneath the rotat¬ 
ing band may be measured by forcing the band 
down against the shell body and measuring the 
resulting deformation of the band. This forcing 
is accomplished by placing the shell in a com¬ 
pression machine with its band between two 
diametrically opposite shoes and applying a pre¬ 
determined load. The investigation at the South¬ 
western Proving Ground has shown that %-in. 
square shoes or anvils, with faces curved to fit the 
rotating band are suitablefor applying the test to 
105-mm shells, and that a load of between 10,000 



Figure 2. Machine used in Klipsch test. 


and 11,000 lb, equivalent to approximately 75,000 
psi on the band, gives satisfactory results. 

In the Klipsch test, the shell is compressed at 
five positions taken at 36-degree intervals 
around the circumference of the band, and the 
resulting deformation determined for each posi¬ 
tion. The machine, which is similar in principle to 
the standard Brinell tester, is shown in Figure 2 
with a 105-mm shell undergoing test. It is of the 
hydraulic type and the upper gauge in the head 
is a pressure gauge which reads the load applied 
to the shoes. A dial gauge for measuring the de¬ 
formation is mounted below the pressure gauge. 

Data obtained with this equipment can be ex¬ 
pressed in terms of either indicated clearance, 
Ci, or indicated looseness, L h Ci is the sum 
of the clearance or gaps between the band and 
its seat at diametrically opposite points. L/ = 
Cj -f- Lo, where 2 L 0 is the difference in actual 
(unpressed) outside band diameter and the 
nominal band diameter of 4.220 in. L/ takes into 
account variations in both band diameter and 
band clearance. 

The machine is very simple to operate, and 
shells are usually handled and tested at the rate 
of 100 per hour, five deformations being taken 
on each shell. The cost of the Detroit machine 
is $2,550.00. 

107 RESULTS 

In Figures 3 and 4, results reported by the 
Southwestern Proving Ground are plotted for 
a large number of shells from different manu¬ 
facturers and different lots, as indicated by the 
code symbols. These shells were all tested in the 
Detroit machine and fired. In Figure 3, the indi¬ 
cated clearance is plotted against muzzle veloc¬ 
ity, and in Figure 4, indicated looseness against 
the same quantity. If an indicated clearance of 
2 mils is selected as an acceptance limit, then, 
as seen in Figure 3, the muzzle velocities of the 
approved shells will range from 1,006 to 1,032 
fps and 28 shells whose muzzle velocities fall 
within the above limits would be rejected as 
unsatisfactory. If indicated looseness is used as 
the criterion for acceptance and the limit be 
chosen as 2.5 mils, then, as seen in Figure 4, the 
approved shells will vary in muzzle velocity from 
1,008 to 1,032 fps and 21 shells with velocities 
within the above limits would be rejected as un- 







METHODS FOR DETECTING DEFECTIVE ROTATING BANDS 


182 

CO 

9 

_l 



8 

z 

7 

UJ 

o 

6 

z 


< 

5 

(T 


< 

UJ 

4 

-I 


o 

3 

Q 


UJ 

2 

1- 


< 

1 

o 

1 

o 

z 

0 


990 




























































ALL VLLUCI 1 Itb UUKKtOItU 
TO 1020 FPS REFERENCE - 





























# 

* 





















•; 

! 




. 




















• 




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• 

.. ; 

:• .... 




S. 

Ms i 

!' " ; 

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. 


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hr 



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• 





1000 


1010 1020 
MUZZLE VELOCITY IN FPS 

Figure 3. Effect of indicated clearance on muzzle velocity. 


1030 1035 


satisfactory. Actually, the percentage of good 
shells rejected in both cases is so small that it is 
negligible. Apparently, therefore, there is little 
to choose from in determining which acceptance 
method to use. The odds are, however, slightly 
in favor of indicated looseness as providing the 
better production test of improperly banded 
105-mm shells. 

When this investigation was started in Janu¬ 


ary of 1944, improper banding was the major 
factor in muzzle-velocity dispersion for 105-mm 
shells. At the conclusion of this investigation the 
dispersion attributable to powder variations had 
become a major factor in the problem. It was 
believed that, with the experience gained in the 
band investigation, real and rapid progress could 
be made in discovering the causes that underlie 
the powder dispersion. 


CO 

2 


CO 

CO 

UJ 

z 

UJ 

CO 

o 

o 


o 

UJ 

s 

o 

o 



990 1000 1010 1020 


MUZZLE VELOCITY IN FPS 

Figure 4. Effect of indicated looseness on muzzle velocity. 


1030 1035 



































































































Chapter 11 


HELIUM-PURITY INDICATORS 

By F. L. Yost a 


INTRODUCTION 

wo types of helium-purity indicators [HPI] 
were developed to determine the percentage 
of air impurity in helium gas. The first indicator 
was for use with fairly large volumes of gas. It 
was based on the fact that the velocity of sound 
in a helium-air mixture varies considerably with 
the percentage of air. The second was for use 
with small volumes of gas. It was a chemical 
method based on a distinct change in color of a 
solution exposed to the helium-air mixture, oc¬ 
curring when the air impurity exceeded a certain 
marginal amount. 

112 VELOCITY-OF-SOUND INDICATOR 
[ VSI] !- 3 

11-21 Introduction 

This project resulted from the need of the U. S. 
Naval Air Station, Lakehurst, New Jersey, for 
a device for testing the purity of helium in air¬ 
ships. The purposes of purity tests are to deter¬ 
mine the amount of helium in the bag when the 
lift is known and to detect the existence of a 
leak in which air diffuses into the bag. The pur¬ 
ity of the helium varies from 97 per cent to as 
low as 88 per cent. When an airship is about to 
start on a long voyage and is therefore carrying 
a maximum gasoline load, the helium should be 
as pure as possible; for shorter voyages, the 
lower purities can be tolerated. 

When this project was initiated two types of 
analyzers were being used at Lakehurst, both 
based on the fact that the thermal conductivity 
of helium is about six times that of the oxygen 
and nitrogen impurities. One type was very 
accurate as a stationary unit where size and 
weight were not objectionable, but portable 
lightweight units proved unsatisfactory. The 
other type gave very accurate results in the 
laboratory, but the outfit was very sensitive, 
a Technical Aide, Division 17, NDRC. 


bulky and expensive, and was not readily adapt¬ 
able for installation in an airship ballonet. 

Military Requirements 

It was desired that an instrument be developed 
which would simply and rapidly measure 0 to 
10 per cent air impurity in helium with an accu¬ 
racy of 0.5 per cent. It was also desired that the 
instrument should be reliable over a tempera¬ 
ture range of —4 to 95 F. It was further desired 
that the instrument be reliable and be capable 
of being operated by a Service man with little 
previous instruction. 

Summary of Development 

The work on this project was done at the Uni¬ 
versity of Pennsylvania under Section D3 of the 
National Defense Research Committee, before 
the organization of Division 17. The method 
developed involved the determination of the 
velocity of sound in helium-air mixtures. Figure 
1 shows the way in which the velocity of sound 
in helium at room temperature varies with the 
percentage of contamination by air. This veloc¬ 
ity is independent of pressure and the effect of 
temperature is independent of composition. 
Thus, the ratio of the sound velocity in a known 
gas mixture to that in an unknown mixture (i.e., 
unknown percentage) at the same temperature, 
but at a possibly different pressure, determines 
the percentage composition of the unknown, 
independent of temperature and pressure. 

The ratio of velocities is equal to the ratio of 
the resonant frequencies of two identical cavi¬ 
ties containing the two gases. 1 A method was 
developed for calibrating a closed resonating 
cavity and associated electric circuits so that, 
for the ranges indicated in Figure 4, the purity 
of the helium could be determined to a few tenths 
of 1 per cent from a single dial reading. The 
cavity is designed to be permanently installed 
in the airship and, ordinarily, to be open for 
passage of the airship’s helium. When a purity 





183 




184 


HELIUM-PURITY INDICATORS 


test is made, the cavity is temporarily sealed to 
form a closed resonator which contains the 
trapped sample. 

Description and Technical 
Information 

Figure 2 shows the resonating tube assembly.- 
A cylindrical tube was used rather than a closed 
Helmholtz resonator (consisting of two bulbs 



PER CENT AIR IMPURITY 


Figure 1. Velocity of sound in helium-air mixture as 
function of percentage of air impurity. 

connected by a short neck) because the former 
allowed more accurate determination of the reso¬ 
nant frequencies. The tube is 42.0 cm long and 
7.6 cm in diameter, with a 0.16-cm brass wall 
closed with a gas-tight flange at each end. To 
these flanges are bolted removable and identical 
cups, each containing a crystal earphone to 
which electric connection can be made through 
a miniature spark plug and through the vessel 
itself. One of the earphones is used as a sound 
generator and the other, as a receiver to detect 
resonance. Near each end of the vessel is a gas- 
tight globe valve. On the outside of the tube 


there is provision for mounting a thermometer, 
which is a necessary adjunct of the instrument. 
Finally, there is a receptacle for taking the cable 
which comes from the electric equipment. 

One test unit was to be attached to each ship. 
The ship’s gas would communicate at all times 
with the vessel so that unless stratification prob¬ 
lems arose there would be no question of flush¬ 
ing, and the tube would always be treated uni¬ 
formly. Samples were to be taken from the bot¬ 
tom of a ship, as was usual practice. 

The electric equipment, shown in Figure 3, 
consists of three parts: (1) an oscillator which 
generates frequencies between 1,600 and 2,450 
c, with the sound-generating earphone placed in 
the output circuit (the frequency range em¬ 
ployed means that the tube is operating on its 
second harmonic) ; (2) an amplifier, connect¬ 
ing directly with the detector earphone; and (3) 
a peak-sharpening circuit, containing the meter 
which is used to indicate resonances. 2 ” 3 

It was desired to develop an oscillator with a 
range of frequencies not dependent on tempera¬ 
ture and which could be calibrated absolutely. 
Dependence of frequency on temperature is ordi¬ 
narily due to inductance in the circuit. Accord¬ 
ingly, this oscillator was designed to employ 
only resistances and capacitances. It was origi¬ 
nally hoped that resistances and capacitances 
which were unaffected by temperature (over the 
desired range) could be used. However, this was 
not accomplished. It was impossible to use silver- 
mica condensers and advance wire-wound re¬ 
sistors, except where absolutely essential. The 
battery voltages and the paper condensers and 
carbon resistors actually used varied with the 
temperature, so that the frequency of the oscil¬ 
lator did depend somewhat on temperature. 

To save tubes, a double triode is used as two 
stages of the three in the amplifier. Large 
amounts of power are not required, and conse¬ 
quently voltage amplification is the sole consid¬ 
eration, which makes possible the use of small 
battery-operated tubes. 

Without a peak sharpener it is possible to 
make measurements to a tenth of one per cent. 
The peak sharpener permits fixing the position 
of the resonant frequency much more accu¬ 
rately; and, consequently, impurities may be 
measured with greater ease. Moreover, the 






















VEL0C1TY-0F-S0UND INDICATOR [VSI] 


185 



Figure 2. Resonating tube assembly for velocity-of-sound indicator. 


sharpener has the further desirable property 
that background noises, either from the exterior 
or from direct transmission down the tube, are 
greatly reduced with respect to the main reso¬ 
nances. As a result, only true resonances are evi¬ 
dent as the frequency is varied. 

The instrument was calibrated by introducing 
helium-air mixtures of known composition and 
temperature. 0 ’ Data were taken for a curve of 
oscillator dial readings against percentage of 
purity at a given temperature. These data were 
extended to other temperatures by the well- 
known proportionality of the velocity of sound 
to the square root of the absolute temperature. 
The curves thus obtained, shown in Figure 4, 
were checked at temperatures between —20 F 
and 110 F at several different purities. Excellent 
agreement was found, provided care was taken 
to thermostat the tube for a few minutes before 
each reading. 

The above calibration was made at room tem¬ 
perature. However, in the field, the electric 


equipment will be at nearly the same tempera¬ 
ture as the tube and the gas, and, accordingly, 
tests were made on the behavior of the circuit 
at high and low temperatures. In a range of tem¬ 
perature from a few degrees below 0 to 110 F, 
the change in frequency of the oscillator 
amounted to 9 c at a frequency of 2,000 c, which 
would mean an error of approximately 0.2 per 
cent in the purity determination. Most of this 
change is due to a reduction of the battery volt¬ 
age at low temperatures. With such a small effect 
it is quite permissible to interpolate the correc¬ 
tion on the calibration curves, and this was done. 

The presence of water vapor in the gas could 
only affect the purity determinations if the gas 
were warm and nearly saturated. Since water 
vapor could only leak in with the air, there could 
hardly be enough present to impair the accuracy 
of the instrument. 

Several types of tests were considered before 
it was decided to use the velocity of sound as a 
test of purity. Those involving thermal conduc- 


















































186 


HELIUM-PURITY INDICATORS 


o.i 



00005 



90K: IOK 


0.2 M B, 



K = 10°OHMS 
M = I0 6 0HMS 

W= ADVANCE WIRE WOUND 

S = SILVERED MICA 

E | ,E 2 = CRVSTAL ear phones 

CAPACITIES IN yuf 

A'= + 1.5V TO GROUND 

B, = + 90V TO B 1 

B 2 = + 45V TO B 1 

C| - t4.5 V TO GROUND 

C 2 = -1.5V TO GROUND 

Cy -4.5 V TO GROUND 


DETECTOR S AMPLIFIER 


L 





-WWV~ o 




. 

v* — 


i 2.0 M 


'2 M i 


3>: 


-200 

AMP 


W> 0.26 M 


SHARPENER , INDICATING METER, 
AND BATTERY CHECK 


15 K 

p/WW- 


3 2 


0.9 M 


90 K 


D<J? oB ' 

V 5 


A B 1 
o o 

a 


SWITCHES 


Figure 3. Circuit diagram for velocity-of-sound indicator. 


PER CENT PURITY 



20 30 40 50 60 70 80 90 IOQ 

DIAL READING 

Figure 4. Percentage purity of helium as function of dial reading and Fahrenheit temperature. 




































































































CHANGE-OF-COLOR INDICATOR [CCI] 


187 


tivity had already been used at Lakehurst and 
hence were not considered further. Methods 
based on measurement of viscosity, dielectric 
strength, and specific heat appeared to be inher¬ 
ently inaccurate for various reasons. Methods 
based on the absorption of sound or light (infra¬ 
red or ultraviolet) seemed too difficult to be set 
up in the field. Other methods, based on measure¬ 
ment of density, velocity of sound, rate of dif¬ 
fusion, index of refraction, etc., appeared to be 
adaptable for precision field work and of these 
the velocity of sound was chosen as the most 
promising. 

113 CHANGE-OF-COLOR INDICATOR [CCI] 4 
Introduction 

The VSI described above required a relatively 
large volume of gas for a determination. A de¬ 
vice suitable for measuring the purity of small 
volumes of helium, such as are used in range 
finders, was also needed. The NDRC was re¬ 
quested to undertake the development of such a 
device, and the CCI resulted. 

Military Requirements 

The device was to operate up to about 10 per 
cent air contamination with an accuracy of 
about 10 per cent in this figure. An additional 
requirement was that it be very small—in one 
application because of space and mechanical lim¬ 
itations, and in another because of weight 
limitations. 

Summary of Development 

In studying the problem, a number of gas and 
contamination indicators were investigated in 
the trade literature. Many commercial devices 
used simply a vial or ampule which can be 
broken, the contents of which change color to 
indicate the presence of the gas it was designed 
to detect. A similar scheme was developed by the 
Gulf Research and Development Company 4 for 
detecting by chemical means air (actually oxy¬ 
gen) contamination in helium. 

The method developed does not actually deter¬ 
mine the percentage contamination but rather 
indicates whether or not the contamination ex¬ 
ceeds a specified value. The equipment for such 


a purity determination consists of a small glass 
vial, with enclosed liquid, attached to the ap¬ 
paratus to be tested by means of a small rubber 
hose which can be constricted with a clamp. 

Description and Technical 
Information 

The arrangement for a purity determination 
is shown in Figure 5. The screw clamp is placed 
loosely at a mark on a small-bore rubber tube, 
and the sealed tip of the vial is slipped into the 



Figure 5. Change-of-color indicator assembly pre¬ 
pared for purity test. 


long end of the tube up to the bulb’s shoulder. 
Another section of auxiliary rubber tube is 
attached to the helium-filled apparatus. (The 
fittings shown in Figure 5 are for a height 
finder.) The needle valve is then opened, a small 
quantity of gas is sucked from the apparatus to 
flush the air out of the valve and the needle 
valve is quickly closed. The auxiliary rubber 
tube is removed, and the first section of tube with 
the bulb attached is connected to the apparatus. 
The needle valve is opened again, and the tip of 
the vial is broken inside the rubber tube. Two 
or three minutes are allowed for the bulb to fill 
with gas from the apparatus. The rubber tube 















188 


HELIUM-PURITY INDICATORS 


is then closed off by means of the screw clamp 
and the needle valve is closed. 

The contents of the vial are agitated for about 
two minutes. The reddish brown solution in the 
vial goes through a series of color changes, the 
final color depending on the amount of air pres¬ 
ent. With small amounts of air, the solution 
changes to dark green; with larger amounts, to 
light green; and when the specified limit is 
reached the final appearance is milky white. If 
the limit is exceeded, the final change from light 
green to white occurs quite sharply. 

The vials have been made to indicate nomi¬ 
nally 10 per cent air contamination and can be 
depended on to give an end point at from 9.5 to 
10.5 per cent air under ordinary conditions. For 
other conditions, there can be supplied a table 
showing the percentage of contamination result¬ 
ing in milky white color for pressures between 
800 and 600 mm Hg and temperatures between 
—30 and 50 C. The corresponding percentages 
of contamination vary from 7 to 17.5 per cent. 

The indicator solution used is made by adding 
5.26 grams of anthraquinone and 1.16 grams 
of sodium hydrosulfite to a mixture of 250 ml 
of 95% ethyl alcohol (boiled) and 250 ml of 0.5 
normal sodium hydroxide. Air is excluded dur¬ 
ing the operations by putting the dry powdered 


materials in a stoppered flask, evacuating the 
vessel, and then allowing the mixture of alcohol 
and sodium hydroxide to run into the flask 
through a separating funnel. 

The vials are filled with the proper quantity of 
solution to indicate 10 per cent air in the helium 
under normal conditions of temperature and 
pressure. The solution is adjusted so that no cor¬ 
rection is required for the small amount of air 
present in the rubber connection to the appara¬ 
tus. The air in each bulb is displaced with butane 
before the required amount of reagent is put in, 
after which each vial is evacuated and sealed. 

For maximum precision, it is necessary to take 
into consideration any air which may be allowed 
to get into the vial during the test. The inclusion 
of a small amount of air is unavoidable, but the 
test procedure has been designed so as to keep its 
volume constant and small; and the necessary 
adjustment has been made in the solution to 
compensate for it. In the particular rubber con¬ 
nection with which the vials are used there is a 
constant volume of 0.34 ml of air admitted. 

A number of these vials were made and fur¬ 
nished to the Naval Bureau of Ordnance, the 
Coast Artillery Board, and the Field Service 
Maintenance Division of the Army Ordnance 
Department. 





Chapter 12 


BATTLE NOISE REPRODUCTION FOR TRAINING AND 
SCREENING BATTLE PERSONNEL 8 


T he object of this project was the develop¬ 
ment and furnishing of one sample of a 
loudspeaking system of adequate fidelity and vol¬ 
ume for reproducing faithfully the noise of naval 
battle. The contemplated uses of the equipment 
were: Marine surface training, battle noise con¬ 
ditioning, teaching men to speak up or use 
prompt visual observance, psychiatric screen¬ 
ing and talker training. The development work 
of the project was carried out by the Western 
Electric Company. Naval personnel actively co¬ 
operated in the making of recordings and Para¬ 
mount Studios assisted in re-recordings. 

The reproductions were to be in the form of 
sound films. It was originally decided to make 
three distinct demonstration films: (1) an iden¬ 
tification sequence with a commentary explain¬ 
ing the nature of each individual sound being 
reproduced; (2) a short jungle combat sequence; 
and (3) an attack on an aircraft carrier. Items 
(2) and (3) were to be composed mainly of the 
individual sounds appearing in the identification 
sequence. 

The reproducing system is composed of three 
major units, each housed in a separate truck. 
Unit 1, the reproducing truck, contains the film 
reproducing machines, motor system, all ampli¬ 
fiers, and control facilities. Unit 2, the loud¬ 
speaker trailer, is a truck trailer containing the 
loudspeakers, horn field rectifiers, and cables 
for connection to the reproducing truck. Unit 
3, the power truck, contains a gas-engine-driven 
generator which supplies 220-v, 3-phase, 60-c 
power for the entire system. Power cables for 
connection to the reproducing truck are carried 
on the reels in the power truck. These three units 
may be operated in close physical proximity, the 

a Time and personnel were not available for prepara¬ 
tion of the usual sort of Summary Technical Report of 
this work. The material reproduced here is taken essen¬ 
tially verbatim from the contractor’s final report. This 
condensed report is intended to give an idea of the nature 
of the work and to call attention to the detailed contrac¬ 
tor’s reports associated with it. 


only precaution being that the reproducing and 
power trucks be located behind the loudspeaker 
array. 

The system is designed to provide reproduc¬ 
tion from 200-mil push-pull sound-film records, 
with or without a frequency-modulated control 
track. It will reproduce disk records of the ver¬ 
tical or lateral type, either 33% or 78 rpm, and, 
in addition, provision is made for operation of 
the system from direct microphone pickup. 

The frequency-response characteristics of the 
entire amplifier system are effectively uniform 
from 50 to 10,000 cycles. The electric output of 
the system is 2,400 w and the harmonic content 
at full output is approximately: 17 per cent at 
50 c, 12 per cent at 100 c, 6 per cent at inter¬ 
mediate frequencies, increasing again to 10 per 
cent at frequencies above 7,000 c. For lower out¬ 
puts, say 800 to 1,000 w the distortion is reduced 
to about 5 per cent for all frequencies. These 
distortion components are contributed by the 
final power amplifier. 

The loudspeaker array provides an acoustical 
response effectively uniform for frequencies 
from 50 to 7,000 c, and has a usable range from 
30 to 10,000 c. Loudspeaker efficiencies of 50 per 
cent are achieved, and for full system electric 
output, a sound field of 130 db at 100 ft on axis 
is realized. The dynamic volume range of the 
system when operated from sound records on 
film using control track is 80 db. Approximately 
92 per cent of the total sound energy is radiated 
from the mouths of the loudspeaker array, the 
remaining 8 per cent being radiated from the 
rear. In general, the directional radiation of the 
array is confined to a total angle of 30 degrees. 
Distribution is complicated by the fact that the 
array is not a point source, that ground reflec¬ 
tion occurs in various ways, depending upon ter¬ 
rain, and that the higher frequencies are attenu¬ 
ated more rapidly in air than are the lower fre¬ 
quencies. In addition, as would be expected, wind 
has a marked effect upon distribution and sound 


189 



190 


BATTLE NOISE REPRODUCTIONS FOR TRAINING COMBAT PERSONNEL 


field intensity. Listening tests indicated that the 
system could be heard over flat desert terrain 
for 2.5 miles on axis, with slight crosswind when 
the relative humidity was 10 to 15 per cent. 

The technical details of the system are given 
in the contractor’s progress report. 1 

All original recordings were made at a Naval 
or Marine Corps establishment in the Eleventh 
Naval District Area. Naval personnel were as¬ 
signed to assist the recording group in obtaining 
the particular sounds desired at the various loca¬ 
tions. Recordings were made at the Destroyer 
Base at San Diego, at the Pacific Beach Anti- 
Aircraft Training Center, at Miramar Landing 
Strip, at the Marine Camp at Camp Gillespie and 
at Camp Pendleton. 

The actual editing and cutting for re-record¬ 
ing was governed to a large extent by the orig¬ 
inal film records obtained under actual field re¬ 
cording conditions. All sound tracks to be used in 
re-recording the final negative were organized, 
cut for proper timing, etc., before actual re¬ 
recording was done. 

The aircraft carrier attack needed a very com¬ 
plete sound record. To the normal background 
sound of sea and wind was added the firing of 
several types of antiaircraft guns, the sound 
of approaching planes, individually and in 
groups, the machine-gun fire of those planes and 
finally the explosions caused by bombs and tor¬ 


pedoes. The climax of this sequence required the 
combination of twelve separate sound tracks. 

The sequence depicting jungle warfare was 
composed of much simpler elements. Although 
the climax demanded the use of nine separate 
tracks most of the sequence was handled by com¬ 
bining three or four. It was principally the re¬ 
sponsibility of timing to make the rifle and ma¬ 
chine-gun fire, mortars and grenades sound as 
though they were being used in combat. 

The recording of the above sequences was done 
at the Paramount Studios. 

The equipment was delivered to the Navy in 
January 1944. It was planned that the system 
should be used concurrently in connection with 
two primary programs—the study of the psy¬ 
chological effect of battle noise on personnel and 
the training of Navy personnel under simulated 
battle conditions. 

In connection with this work it was decided 
that additional recording should be made. These 
were made at the Anti-Aircraft Training Cen¬ 
ter, Dam Neck, Virginia; on the USS New York 
on training cruise; at the Naval Proving Ground, 
Dahlgren, Virginia; and at the Amphibious 
Training Base, Coronado Island, San Diego, Cali¬ 
fornia. A complete list of the recordings taken is 
contained in the contractor’s report, 2 as well as 
the details of four thirty-minute battle sequences 
derived from recordings. 


VT I .VI 17* ET 



Chapter 13 


SOUND SPECTRUM OF ORDNANCE EQUIPMENT AND BATTLE NOISES* 


T his project was begun on April 1, 1942, on 
the recommendation of the General Develop¬ 
ment Laboratory of the Signal Corps. The objec¬ 
tive was to study sounds from ordnance equip¬ 
ment, to obtain information about the frequency 
composition of sounds originating from various 
weapons, the intensities of these sounds, the 
phase relationships of the various components, 
and any other information about the sounds 
which might be useful in connection with sound 
ranging. It was felt that as complete as possible 
a knowledge of these sounds was essential in 
order that available sound-ranging equipment 
might be used to its fullest capabilities and 
that better apparatus might be designed. The 
work was performed by Western Electric Com¬ 
pany under Army Service Project SC-27. 

The study included sounds from machine guns, 
mortars, and various field-artillery weapons 
ranging in size from the 105-mm howitzer to 
the 240-mm howitzer. Under battle conditions 
noise may interfere with the operation of sound¬ 
ranging equipment. Information about the char¬ 
acter of interfering sounds is therefore desir¬ 
able. A complete investigation of all possible in¬ 
terfering noises could not be made, but a partial 
investigation was made in which the sounds 
from a number of army vehicles were studied. 

The initial interest was in the sound spectra 
produced by .30- and .50-cal machine guns. High- 
quality phonograph recording equipment was 
assembled and taken to Aberdeen Proving 
Ground where recordings were made of the 
muzzle waves and ballistic waves from these 
guns at various distances. The records were sub¬ 
sequently reproduced electrically, and by means 
of wave filters a determination was made of the 
frequency spectra of the sounds. 1 

Although the recording system used for the 

a Time and personnel were not available for prepara¬ 
tion of the usual sort of Summary Technical Report of 
this work. The material reproduced here is taken essen¬ 
tially verbatim from the contractor’s final report. This 
condensed report is intended to give an idea of the nature 
of the work and to call attention to the detailed contrac¬ 
tor’s reports associated with it. 


machine-gun tests was the best available at the 
time the response was limited on the low-fre¬ 
quency end to 40 c. Later, when it was requested 
that larger-caliber guns be investigated it was 
realized that the original recording system 
would be inadequate. A recording system was 
therefore developed which had a very wide fre¬ 
quency-response range, uniform from 0.1 c to 
several thousand cycles per second. This system 
was practically free from amplitude and phase 
distortion over its operating range. 

With this system recordings were made of 
various guns and howitzers at Fort Bragg 
through the cooperation of the Field Artillery 
Board. Field-artillery weapons ranging in cali¬ 
ber from the 105-mm howitzer to the 240-mm 
howitzer were studied. Additional tests of ma¬ 
chine guns were also made. The results of these 
various tests were given in the contractor’s final 
report. 4 

With this same recording system studies were 
made of the sounds from 60-mm, 81-mm, and 
4.2-in. mortars at various distances up to 2,000 
yd. The results of this study 3 showed why sound 
ranging on light mortars is difficult. At distances 
at which sound ranging is customarily con¬ 
ducted the intensity of the sound from the 
60-mm mortar was found to be about the same 
as that of the ambient noise. The energy in the 
mortar sounds is concentrated at a higher fre¬ 
quency than for the field-artillery weapons and 
lower than for the machine guns. 

A study was made 2 of noises from a number 
of typical Army vehicles ranging in size from 
the jeep to an 8-ton truck and including some 
tractors. 

The reports in the bibliography constitute a 
complete record of the studies conducted under 
this project. The following is an abstract of the 
final report. 

1. A recording system was developed having 
the exacting performance characteristics needed 
to record faithfully the sounds from various 
field-artillery weapons. With this system almost 
500 records were made at Fort Bragg. Analyses 


iiji I, H1L 


191 



192 


SOUND SPECTRUM OF ORDNANCE EQUIPMENT OF BATTLE NOISES 


of 110 selected typical records were made with 
an Henrici analyzer. The complete results are 
given in OSRD 4594. 4 To supplement these 
analyses about 300 of the original oscillograms 
are appended. 

2. The analyses of the spectra of muzzle waves 
from weapons of large caliber (105 mm to 240 
mm) show that at distances of interest in sound 
ranging, the major part of the energy lies in 
the frequency range from about 2 to 40 c. 

3. The ballistic wave, originating at the pro¬ 
jectile in flight, has a sound spectrum which can 
be readily differentiated from spectra of sounds 
originating in other ways. The analyses of the 
ballistic waves do not always show that the maxi¬ 
mum energy is concentrated within a limited 
region. When the major energy is so concen¬ 
trated, the region is located much higher in fre¬ 
quency than for the muzzle wave originating at 
the gun. Sometimes the maximum energy is at 
about 100 c. 


4. The spectrum of the sound produced by the 
explosion of a shell is similar to that of a muzzle 
wave from a large-caliber weapon. On visual in¬ 
spection of the oscillograms, however, high- 
frequency wavelets are evident, preceding the 
main wave caused by the explosion of the shell. 
These oscillograms are therefore readily iden¬ 
tified as being shell sounds. 

5. The “frequency” of the region of major 
energy for the muzzle waves is influenced at 
the source by such factors as the weight of the 
charge of powder used to propel the shell and the 
dimensions of the gun, and is affected in its prop¬ 
agation by the distance and the terrain between 
the gun and the microphone. 

6. Additional data on machine-gun sounds 
confirm earlier findings. 1 The better perform¬ 
ance characteristic of the recording system used 
in the later tests confirmed the presence of the 
low-frequency energy, as suspected, and showed 
its magnitude. 



Chapter 14 


D3 PROJECTS REPORTED BY DIVISION 17 

By J. S. Coleman a 


INTRODUCTION 

ERTAIN OF the projects completed under 
Section D3 of the NDRC prior to the or¬ 
ganization of Division 17 were of such nature 
that had Division 17 been in existence at the time 
of their inception they would have been assigned 
to it. References to these projects were made in 
the initial Bi-Monthly Summaries of Division 
17. Since these projects will not be reported 
elsewhere, resumes of them are included in the 
Division 17 Summary Technical Report. In con¬ 
nection with each, reference is made to a detailed 
bibliography. The work on all these subjects was 
conducted during 1941 and 1942; and these 
brief reports are written to indicate the status 
of each at the end of 1942. 

142 THERMISTORS 

The term “thermistor” is a contraction of the 
words “thermal resistor,” and describes a new 
type of circuit element whose resistance varies 
markedly with temperature. Three classes of 
thermistors are used in electric circuits—the ex¬ 
ternally heated units, the indirectly heated, and 
the directly heated. The first of these is a simple 
thermistor which is usually made in the form of a 
bead, disk, rod, or strip whose resistance is 
caused to vary by changes in ambient tempera¬ 
ture. This type finds application in the measure¬ 
ment or control of temperature, the detection 
and measurement of radiant energy or as a 
temperature-compensating device in electric cir¬ 
cuits. The second, or indirectly heated type, con¬ 
sists of a thermistor unit in thermal contact with 
a heating coil. By controlling the power supplied 
to this coil, the thermistor resistance can be 
made to vary over a wide range. This type finds 
use as an indirectly or remotely controlled cir¬ 
cuit element in telemetric or automatic-control 
functions. The third type of thermistor is di¬ 
rectly heated by the power dissipated in it by 
the current flowing through it. Because the ther¬ 


mistor materials have large negative tempera¬ 
ture coefficients of resistance, a unit of this last 
type has the property of “negative resistance.” 
If the current through such a unit be increased 
beyond an initial critical value, the voltage 
across the unit will decrease with increasing 
current. This phenomenon is due to the rapid 
decrease in resistance resulting from the power 
dissipated as heat in the unit. 

It was, perhaps, the directly heated variety of 
thermistor which provoked the greatest interest. 
Along with their property of dynamic negative 
resistance, units of this type have the additional 
property of inductance, due to the lag introduced 
in the electric response while the unit reaches 
thermal equilibrium. Thus the static-response 
curve may differ radically from the dynamic 
characteristic as the thermal lag prevents the 
effective temperature from following rapid 
changes in current, so that at high frequencies 
the thermistor behaves as an ohmic resistance 
whose value is determined by the total dissipated 
power. Higher response speeds may be obtained 
by reducing the thermal capacity and increasing 
the rate of thermal loss. The limitation in speed 
of response and the dependence on ambient tem¬ 
perature proved the principal difficulties in the 
course of this project. 

A number of specialized uses for thermistor 
devices had been discovered and applied by Ger¬ 
man and American laboratories and an even 
greater number had been suggested by the 
unique thermistor properties. It was in order to 
investigate the feasibility of some of these sug¬ 
gestions and to determine the conditions of oper¬ 
ation under which they might be profitably em¬ 
ployed that a thermistor program was started 
shortly after the formation of the NDRC in 
1940. As a result of these studies it was decided 
that for the applications investigated, with one 
or two exceptions, the thermistor in its existing 
form could not be considered satisfactory for 
service under military conditions, as operation 
is generally critical with ambient temperature 

193 


Assistant Director, Summary Reports Group. 








194 


D3 PROJECTS REPORTED BY DIVISION 17 


unless the device is thermostated or otherwise 
compensated. The applications included the use 
of a thermistor as an amplifier, tuned filter, sine- 
wave oscillator, precise time delay, means for 
self-balance of bridge circuits, current relays, 
and fluid-flow meters. Unless the device is itself 
used to measure, or is controlled by tempera¬ 
ture, it must be compensated, either electrically 
or thermally, for stable precise operation. Un¬ 
fortunately in these cases this compensation was 
either inconvenient or not possible to accom¬ 
plish. 

It is not implied that thermistors are unsatis¬ 
factory for all applications under these condi¬ 
tions of wide variation of temperature. Many 
functions are being well served by their use. 
These functions, however, being simpler and 
better suited to utilize the characteristics of the 
thermistor, were established and, in many cases, 
practiced prior to this program. Such functions 
include current and voltage regulators, surge 
and transient discriminators, remotely con¬ 
trolled resistance units, resistance-capacity os¬ 
cillators, and many others. This NDRC project, 
in addition to defining and extending the condi¬ 
tions of use for these functions, developed the 
additional functions of telemetric temperature 
indicators and sensitive bolometer or radiometer 
elements. 

The former of these two provided a simple 
reliable means for the measurement of tempera¬ 
ture at some remote spot. Suggested applications 
were temperature indicators for aircraft cylin¬ 
der heads, temperature-controlled modulators 
for radio-sounding meteorological balloons, clin¬ 
ical thermometers, oil-temperature indicators, 
and others. Scales can be adjusted to give any 
degree of accuracy over any temperature range 
up to several hundred degrees centigrade. The 
second of these developments, that of the bolom¬ 
eter, has already made remarkable contribu¬ 
tions in the field of detection and measurement 
of infrared radiation. By means of the sensitive 
thermistor as a detecting unit it has been pos¬ 
sible to more than double the sensitivity and 
speed of response previously possible with metal- 
strip bolometers. Although this development is 
described more fully in the Division 16 Summary 
Technical Report, 50 it is of interest to note that 
a sensitivity of 10~ 9 w of incident radiation over 


the band 5 to 20 microns has been obtained with 
a response speed of better than 10 -2 second. This 
is of real importance not only to military prob¬ 
lems of detection and communication but also to 
the wide field of infrared spectrometry, vital in 
the analysis and control of synthetic rubber 
manufacture and other war-industry products 
and processes. 

By the time of the entry of the United States 
into the war, the more urgent need for work of 
other nature resulted in the abandonment of all 
of the thermistor-circuit investigations. Al¬ 
though it is possible that further effort might 
have expanded their fields of use, it is believed 
that other projects, promising more immediate 
and vital returns, justified the diversion of ef¬ 
fort. When it is again possible to take up the 
problems of thermistors, it is anticipated that 
their use will be widely expanded. The develop¬ 
ment of a mechanically stable, thermostated, 
high-speed thermistor, capable of dynamic 
operation over the audio-frequency range, will 
make possible the design of extremely high-Q 
filters with great savings in size, weight, and 
cost. It is probable that new designs, incorporat¬ 
ing grid connections for local heating, and giv¬ 
ing amplifier action without expenditure of fila¬ 
ment power, will be devised, along with many 
other novel and specialized forms. It is particu¬ 
larly recommended that the engineering 
branches of the Air Services, with their severe 
problems of temperature compensation of in¬ 
struments, take every opportunity to benefit 
from the use of thermistors. 

The work on thermistors was conducted un¬ 
der a number of project headings and by a 
variety of contractors, as follows: 

Thermistors 

Carnegie Institute of Technology 1 ’ 2 
Massachusetts Institute of Technology 20 21 
Rensselaer Polytechnic Institute 33 - 34 
Yale University 46 48 

High-Speed Thermistors—Harvard Univer¬ 
sity 12 ' 14 

Thermistors in Connection with Tempera¬ 
ture Control—University of Minnesota 42 ' 43 

Methods of Compensating Thermistors and 
Thermistor Circuits for Ambient Temper¬ 
ature Variations—University of Minne¬ 
sota 44 ' 45 

tfmAL 



DEVELOPMENT OF TELEMETRIC FLOWMETERS 


195 


Applications of Thermistor Units for Appli¬ 
cations Designed by the Chairman of Sec¬ 
tion D3 b —Harvard University 1519 

Thermistors as Trigger Amplifiers—The 
Franklin Institute 3 ' 5 - 49 

Thermistor Bolometer—Northwestern Uni¬ 
versity 31 - 32 

Application of Thermistors to Submarine 
Mines—Princeton University. 

143 DEVELOPMENT OF TELEMETRIC 
FLOWMETERS 

The development of a thermistor flowmeter 
was briefly mentioned in Section 14.2. A flow¬ 
meter is important for the operation of long- 
range aircraft in that it provides an instanta¬ 
neous check on the rate of consumption of 
fuel. At the time that this project was begun, 
there was no available aircraft flowmeter 
which could combine the required accuracy 
with reasonable specifications of reliability, 
size, and power consumption. The thermistor, 
having an extremely high temperature coeffi¬ 
cient of resistance, held some promise as a 
Pirani-gauge type of flowmeter. Therefore, an 
attempt was made by the Gulf Research and 
Development Company, 610 to design a thermis¬ 
tor flowmeter which would not only indicate 
rates of flow between 30 and 300 gallons per 
hour with an accuracy of ±3 per cent but 
would also provide this accuracy over the tem¬ 
perature range of at least —20 to +30 C. Con¬ 
siderable effort was expended on this program 
before it was finally decided that, while the 
thermistor unit had adequate sensitivity, it was 
not feasible to secure the necessary accuracy 
over the wide ranges of temperature required 
by operational specifications. 

The contractor next turned to the most prom¬ 
ising commercial development in flowmeters 
with the idea of improving its perfomance to 
meet the specifications set by military require¬ 
ments. The unit selected 11 was a rotor-type flow¬ 
meter comprising a screw or spiral-bladed rotor 
revolving concentrically in a cylindrical non¬ 
magnetic-steel measuring section whose inside 
diameter was approximately that of the flow line 
in which it was to be inserted. The rotor, slightly 

*> AC-34. 


over 1 in. in diameter, was free to rotate on ball 
bearings about the axis of the flow line with 
small clearance between the rotor and the 
measuring section. Integral with the rotor was 
a small magnet with its poles displaced from 
the axis of rotation. A pickup coil and mag¬ 
netic circuit were arranged opposite the rotor 
and outside the measuring section in such a 
way that an a-c generator was formed. The 
output of this generator varied in frequency 
and voltage with the speed of rotation of the 
rotor and consequently with the rate of fluid 
flow. The indicating element which received 
the a-c output from the metering element was 
a 270-degree-scale rectifying milliammeter 
normally mounted on a panel at a distance 
from the generator. This device in laboratory 
tests could be adjusted to give an accuracy of 
±3 per cent for a single temperature condi¬ 
tion, and approximately ±8 per cent for the 
temperature ranges required. It was found 
that these errors were due to three factors: 
(1) a change in generator output with tem¬ 
perature, resulting from the change in per¬ 
meability of the rotor magnet and the stator- 
field materials; (2) the increased slippage of 
the rotor with increased rates of fluid flow, 
and the thermal expansion of the fuel; (3) the 
change in calibration of the indicator instru¬ 
ment with temperature. Each of these factors 
contributed an additive error of between 2 
and 3 per cent, indicating values lower than 
actual with increasing temperature. 

It was found possible to decrease two of these 
errors by introducing straight flow-line con¬ 
nections into and leading from the metering 
rotor and by the development of a frequency- 
indicating meter which was not temperature 
sensitive. Further, it was learned that much 
of the slippage error due to the expansion of 
the fuel with increased temperature could be 
reduced if mass rate of flow rather than vol¬ 
ume flow (giving fuel consumption in pounds 
rather than gallons) were considered. Recom¬ 
mendations based upon these experimental find¬ 
ings were turned over to the commercial manu¬ 
facturer and military agencies. Best results 
obtained with this corrected commercial instru¬ 
ment showed that a mass-rate-of-flow accuracy 
of ±2 per cent over a temperature range of 


rONFTPRNTT AL. 




D3 PROJECTS REPORTED BY DIVISION 17 


196 


—20 to +110 F could be realized. Further, this 
gain was accomplished without requiring addi¬ 
tional size, weight or power consumption. With 
specifications met, this project was terminated 
in August 1942. 

144 STRAIN GAUGES" 

Following expressions of interest on the part 
of Army and Navy aircraft-testing sections, a 
program leading toward the development of 
improved wire strain gauges was instituted in 
January 1941 under contract with the Univer¬ 
sity of Pennsylvania. 3 " 41 At that time three 
types of gauges were used for measuring struc¬ 
tural strains—magnetic reluctance, optical lever 
and resistance wire strain. None of these 
was completely satisfactory. While the mag¬ 
netic gauges possessed the advantage of large 
output without amplification, they were unde¬ 
sirable with regard to linearity, temperature 
compensation and difficulty of mounting. The 
optical gauges, having adequate sensitivity, 
were difficult to mount, lacked temperature 
compensation for some applications, gave re¬ 
sults which were not generally reproducible, 
and were almost impossible to telemeter. Exist¬ 
ing designs of wire gauges were principally 
criticized on the basis of lack of temperature 
compensation, low output, impossibility of re¬ 
covery after measurement for recalibration, ex- 
pendability, and high cost. 

On the basis of these and other criticisms of 
existing designs a set of specifications was pre¬ 
pared by the Materiel Division, Wright Field, 
USAAF, to cover the design of an ideal wire 
gauge. These specifications were: 

1. The gauge should reproducibly record 
strain up to ±0.5 per cent, with a precision of 
1 per cent of the maximum strain. 

2. The gauge should be linear over the range 
of strain to be recorded. 

3. The calibration constant of the gauges 
should be independent of temperature from 
—40 to 140 F. Strains resulting from thermal 
expansion of the member under test should not 
be recorded. 

4. The gauge should be less than 1.5x0.5x0.75 
in. in size and should weigh less than 1 oz. 

c AC-20. 


5. The power consumed by the gauge should 
be less than 50 mw. 

6. The output of the gauge for a strain of 
0.5 per cent without auxiliary amplification, 
other than that obtained with an output trans¬ 
former matched to an impedance of 0.5 
megohm, should be not less than 5 v. 

7. The gauge should be demountable in less 
than 15 minutes for the purpose of rechecking 
the calibration and should be remountable in 
a corresponding length of time. 

8. The attachment of the gauge should not 
damage the surface to which the latter adheres, 
should not affect the material of which the test 
member is made, nor alter appreciably its me¬ 
chanical impedance. 

The first step in the design of a resistance 
wire gauge to meet these specifications was the 
development of a temperature-compensated 
gauge. This was achieved by using four simi¬ 
lar resistance-wire arms in a Wheatstone 
bridge arrangement. Two diagonally opposite 
arms are cemented to the surface to be studied 
while the remaining two are cemented to a 
strip of the same material which is not strained 
but is placed in juxtaposition with the surface 
in question, so that the temperature of all arms 
is the same and the effect of any thermal ex¬ 
pansion of the member is nil. The advantage 
of this arrangement is that not only is the 
output twice that of one arm alone, but also 
the bridge connection minimizes the effect of 
lead resistance and permits the use of long 
lines. In order to minimize errors resulting 
from flexure and to insure intimacy of contact 
and ease of application, the gauges are made 
by cementing the wire elements to a paper strip 
which is applied directly to the surface to be 
measured with a quick-drying adhesive. 

A number of designs involving various ar¬ 
rangements of bridge arms, papers, and glues 
were made before a satisfactory design was 
reached. This design, while admittedly a com¬ 
promise, was given extensive tests which cor¬ 
roborated its laboratory performances. Two 
models were made—a small unit having the 
compensating arms located above the strain 
arms and a larger standard unit having the 
compensating arms located alongside the strain 
arms. As expected, this latter arrangement 




STRAIN GAUGES 


197 


gives a more exact compensation for tempera- 


ture. The physical specifications of 

the units 

are: 

1. Dimensions. 

Size 

Standard 

Small 

Length 

1.5 in. 

1.3 in. 

Width 

0.8 in. 

0.3 in. 

Height 

0.25 in. 

0.2 in. 

Total weight 

0.14 oz 

0.1 oz 

Effective area 

0.9x0.6 in. 

0.9x0.2 in. 

Distance from test member 

to gauge 

0.003 in. 

0.003 in. 


2. Electric constants. Each arm of the gauge 
consists of 5 y 2 in. of 1-mil lacquered con- 
stantant (advance) wire laid in six zigzags 
about % in. long, with y 32 - in. separation, giv¬ 
ing an arm (and bridge) resistance of approxi¬ 
mately 140 ohms. (See Figure 1 of D3 174. 39 ) 
With 45 mw of power furnished each bridge 
unit from a stabilized 2,000-c oscillator, 10 _r> v 
is obtained for a strain of 10~\ An a-c ampli¬ 
fier raises this level to that necessary to drive 
a rectifier-meter indicator giving full-scale de¬ 
flection (100 divisions) for a strain of 70 X 10 -5 . 

3. Linearity. The gauges themselves are 
linear and reproducible to better than ±5 X 10 -5 
strain (i.e., 1 per cent of 0.5 per cent strain). 
The linearity of the oscillator and amplifier are 
designed to be below this figure. 

4. Range. A maximum range of ±10 -2 strain 
(i.e., 1 per cent strain) is possible at frequen¬ 
cies up to at least 30,000 c. There are no de¬ 
tectable humidity or corrosion effects (except 
as they affect the time required for the fixing 
glue to set). 

5. Temperature effects. For different posi¬ 
tions of the dummy arms, these are shown in 
an unnumbered illustration in D3 284. 41 For 
the standard gauge, these all reduced approxi¬ 
mately ± 10 -5 strain from —10 to over 120 F. 
Adding 150-ft leads approximately doubles 
this error. 

6. Tension or compression. It is possible to 
differentiate between compression and tension 
by a phase-sensitive device such as the magic- 
eye tube. 

7. Error due to stiffness of gauge unit. For 
a force of 1 lb, this has been measured to be 
equal to a strain of 3 X 10~ 4 for the standard 
unit and 9 X 1()- 4 for the small. This corre¬ 
sponds to errors of 1.5 per cent and 0.75 per 
cent, respectively, if the units are used on 


% 2 -im duralumin. This error decreases with 
increasing thickness of the test member. 

8. Attaching gauge to member. This has 
been done by using ordinary synthetic adhe¬ 
sives (such as Duco cement) diluted with vola¬ 
tile solvents. With these, a drying time of 4 
to 20 hours is required, depending upon the 
humidity and magnitude and duration of strain. 

9. Recalibration of gauges. The units can be 
removed, for recalibration after a series of 
measurements, by softening the bond with a 
solvent and using a razor blade to detach the 
paper from the test member. However, in view 
of the uniformity with which these units are 
manufactured, this process is believed unnec¬ 
essary and is not recommended. 

10. Effect of long leads for telemetering. 
The balance of the bridge is not affected by lead 
length provided noise pickup does not occur at 
oscillator frequency. However, there is a slight 
temperature effect. 

11. Errors arising from flexure. These 
gauges, in common with most gauges, do not 
distinguish flexure from pure strains. The 
error is a function of the distance of the gauge 
element from the member, and is minimized in 
the case of these units by having the elements 
only 0.003 in. from the test surface. This error 
can be eliminated by putting a gauge on each 
side of the test member, in which case the pure 
strain is given by the sum of the readings and 
the flexure by the difference. 

12. Installation and operation. A section of 
the circuit of an equipment constructed for 
testing of aircraft structures is shown in an 
unnumbered illustration in D3 284. 41 

It is believed that these refinements, al¬ 
though not representing the ultimate in the de¬ 
sign of wire strain gauges, have made it possi¬ 
ble to extend the use of such gauges under a 
wide variety of conditions in laboratory struc¬ 
tures for testing and proving. Being small and 
compact, inexpensive to manufacture, easy to 
attach, insensitive to temperature variations, 
and allowing long lines which permit telemeter¬ 
ing, rapid switching from unit to unit, and the 
application of standard amplifier and indicator 
techniques, they represent a flexible and valua¬ 
ble contribution to the art. Although Section 
D3 recognized that much remained to be done 




198 


D3 PROJECTS REPORTED BY DIVISION 17 


in the way of developing quicker setting ce¬ 
ments and simplified mechanical designs, it was 
felt that these improvements might more logi¬ 
cally be accomplished by the several labora¬ 
tories and the manufacturer. Consequently, 
after the results of this project were communi¬ 
cated to the interested military establishments, 
the project was terminated. 

145 MEASUREMENTS OF STRAIN 

TRANSIENTS ON EXTERNAL SURFACES 
OF GUN BARRELS' 1 

Paralleling the project on the improvement 
of strain gauges, Section D3 was requested by 
Watertown Arsenal to cooperate in a program 
leading to improved methods of simultaneous 
high-speed measurements of strains experi¬ 
enced by gun barrels during firing. It was be¬ 
lieved that the accurate and complete deter¬ 
mination of the magnitude, period, and loca¬ 
tion of these strains would permit improve¬ 
ment in gun design and manufacturing proc¬ 
esses, leading in turn to longer barrel life, 
higher muzzle velocity, and increased produc¬ 
tion. The NDRC project in this case was 
charged with the development of suitable 
measuring apparatus and with assisting in 
making and interpreting the first series of 
measurements on a sample barrel. It was not 
concerned with the collection of data or inter¬ 
preting its significance as it might affect de¬ 
sign or production, this being held the proper 
function of the arsenal technical staff. 

A program was set up calling for the design 
and construction of a complete laboratory 
equipment capable of simultaneous multiple re¬ 
cording of high-speed strain transients having 
minimum periods of 5 X 10 -5 second. The 
equipment, as developed, utilized electric wire- 
type strain gauges (not compensated for tem¬ 
perature) to produce voltages proportional to 
the strains observed by unbalance in a bridge 
circuit. These voltages, after passing through 
bridging and selector circuits, were fed into 
the vertical amplifier of an oscillograph where 
they controlled the deflection of the spot 
on a short-persistence, blue-screen, cathode-ray 
tube. An oscillographic trace was obtained by 


photographing the vertical position of the spot 
on high-speed film attached to the periphery 
of a moving drum which rotated about a ver¬ 
tical axis. A square-wave electronic switch was 
provided to bring the spot into the camera 
field during the measurement period. Four 
such camera oscilloscopes were provided in the 
complete equipment, permitting simultaneous 
recording (through a simple switching ar¬ 
rangement) of 4 of 32 attached gauges. A time 
reference was obtained, by a momentary elec¬ 
tric contact, established by the projectile be¬ 
tween the barrel and a taut wire stretched in 
front of the muzzle, which gave a sharp de¬ 
flection to one of the oscilloscope traces. Means 
were also provided for individual calibration 
for each of the several gauges. 

With this system, it was found possible to 
record strains of the order of 2 X 10~ 6 in. per 
inch having periods as short as 5 X 10 -5 second. 
It was also possible for the first time to study 
the effects of recoil, the expansion and con¬ 
traction of barrels resulting from gas pres¬ 
sures and driving band motion, and the oval- 
izing and vibratory strains, both in tension 
and torsion. The first series of measurements, 
made on a 37-mm field gun, yielded much vak, 
uable information. It showed, for example, that 
the maximum strains resulted from the 
passage of the driving band. This strain takes 
the form of a barrel expansion at the point of 
the driving band, immediately preceded and 
followed by a compression, as well as a rather 
severe and complex vibratory condition, start¬ 
ing at the muzzle with the emergence of the 
projectile. Very little ovalizing deformation 
was observed. For a complete report of these 
measurements see D3 287. 20 

During the course of this project there was 
developed an additional piece of equipment for 
the measurement of internal diameters of bar¬ 
rels under very high hydrostatic pressures. 
This device—an electric micrometer—is based 
on the principle of a wire voltage divider, the 
position of the slider on the resistance wire 
being controlled directly by the internal dimen¬ 
sion of the barrel. The voltage divider is con¬ 
nected in a Wheatstone-bridge circuit, the re¬ 
sistance wire serving as two adjacent arms of 
the bridge with the slider as one of the bal- 


d AC-20. 





THE RADON INDICATOR 


199 


anting contacts. This device gives 0.001-in. ac¬ 
curacy for water or oil immersion tempera¬ 
tures of 20 to 60 C, and with pressures exceed¬ 
ing 10 5 psi. 

After the first measurements with this equip¬ 
ment arrangements were made to turn it over 
to the arsenal laboratory to be incorporated 
into a special laboratory constructed for the 
purpose of continuing measurements on the 
37-mm and other barrels as well as various de¬ 
signs of mounts and carriages. 

146 THE RADON INDICATOR 

One of the serious problems that has con¬ 
fronted industrial health authorities for a num¬ 
ber of years is that of radium poisoning of 
personnel handling radioactive luminous paint. 
As there is no known cure for serious cases 
resulting from overexposure to these com¬ 
pounds, it is of paramount importance to pre¬ 
vent such overexposure. To do this it is neces¬ 
sary to have an instrument which has adequate 
sensitivity to detect the presence of tolerance 
quantities of radon in the breath samples of 
exposed workers. Although such instruments 
were available prior to the inception of this 
project, they were judged unsuitable because 
their low sensitivity and the excessive time re¬ 
quired to make a single analysis precluded the 
possibility of maintaining adequate control of 
worker exposure and poisoning. 

An instrument capable of measuring the 
amount of radon present in an air or breath 
sample serves the double purpose of measuring 
the amount of radium and its decay product 
present in a human body, and measuring the 
amount of radon in room air which may be 
harmful to the shop workers. That such an in¬ 
strument is both necessary and adequate to 
the control of worker radium poisoning is 
firmly established, for the best index of the 
total amount of radium stored in the body of 
a victim is the radon content in the expired 
breath. Medical research has determined that 
the tolerance dosage of radium has been 
reached, when a breath sample from an ex¬ 
posed person contains 10 -11 curie of radon per 
liter of air. The measurement of these con¬ 
centrations is accomplished by counting the 


a particle activity of the sample. Hence, the 
problem undertaken in this project by the 
Massachusetts Institute of Technology 28 30 was 
to develop a practical apparatus capable of 
counting 150 <x particles per hour (10 -11 curie) 
from radon and its decay products to an accuracy 
of 20 per cent in a reasonable period of time. 

The problem was broken down into three 
parts: (1) the design of a rugged sampling 
flask by means of which breath samples and 
room-air samples might be conveniently ob¬ 
tained, (2) the design of a suitable a-particle 
detecting apparatus, and (3) the design of a 
suitable indicating and/or recording equipment. 

The detecting and indicating apparatus de¬ 
veloped consists, essentially, of a sensitive ioni¬ 
zation chamber which feeds pulses into a high- 
gain linear amplifier whose output operates an 
Esterline-Angus pen recorder. The passage of 
an a particle in the ionization chamber is 
recorded as a deflection of the pen recorder, 
thus yielding a permanent record of the event. 
The final apparatus constructed was capable of 
measuring 10 -12 curie of radon in the first two 
hours to an accuracy of 20 per cent. It was 
therefore more sensitive by a factor of 10 than 
called for in the original specifications. This 
additional sensitivity proved of great benefit 
in maintaining rigid control over workers dur¬ 
ing the early periods of exposure. The counting 
range of the apparatus is approximately 50 to 
500 a particles for a two-hour run. 

The complete apparatus, being highly sensi¬ 
tive, requires shock-proof mounting and opera¬ 
tion by a skilled technician. In view of this and 
in order to make radon-analysis service as 
flexible and widely available as possible, two 
types of special sampling flasks were developed 
—one for obtaining samples of breath from pa¬ 
tients, and the second for obtaining samples of 
air from rooms suspected of containing radon. 
The breath-sampling flask consisted of a one- 
liter, pyrex flask with a specially designed two- 
way stop-cock. The device was so arranged 
that when a blowing tube was protruding up 
from the flask, air from the lungs could be 
blown through the flask; and when the blow¬ 
ing tube was turned down, the flask was sealed 
from outside air. The sampling flask, designed 
to collect room-air samples, consisted of a 



200 


D3 PROJECTS REPORTED BY DIVISION 17 


1- liter glass flask with two stop-cocks. This 
type of flask may be evacuated and opened at 
a point where the atmosphere is suspected of 
containing measurable amounts of radon. 

For convenience in shipping and handling, 
the flasks are contained in specially developed 

2- gallon metal cans filled with shock-absorbing 
material having the consistency of hard sponge 
rubber. Rigorous shock and sustained-pressure 
tests have proved the ruggedness of this de¬ 
sign. By the use of a large number of these 
flasks, a single detecting and indicating ap¬ 
paratus is able to provide quick and reliable 


service to a large number of plants and fac¬ 
tories. As of early 1945, three sets of equip¬ 
ment had been constructed for this service, 
which was widely used by military medical 
services, insurance companies, state industrial 
hygiene departments and federal health 
agencies. 

With the design, construction, and success¬ 
ful demonstration of this equipment, Section 
D3 made arrangements for the unit to be taken 
over by a nonprofit laboratory to provide this 
service at a minimum of time and cost to all 
individual companies and agencies requiring it. 





GLOSSARY 


AFC. Automatic frequency control. 

a Particle. Helium nucleus. 

Aperiodic FEI System. A firing error indicator system in 
which the microphone or microphones are highly damped, 
have a frequency flat response characteristic up to about 
10,000 c and are used to reproduce with a high degree of 
fidelity the wave-form profile of acoustic shock waves. For 
other characteristics, see text (Sections 2.5.2, 2.5.3, and 
2.6.7). 

Aperiodic Microphone. See Aperiodic FEI System. 

Apex Angle (of shock-wave cone). The shock-wave disturb¬ 
ance trailing backward from the nose of the bullet has the 
shape of a right circular cone with the trajectory as axis of 
symmetry. The angle a made by any element of this cone 
with the axis is called the apex angle (sometimes called the 
semi-apex angle). This angle is given by sin a = s/v where 
s is the velocity of sound and v the velocity of the bullet. 
Since the H and T discontinuities have velocities respectively 
slightly higher and slightly lower than sonic, the shock- 
wave cone referred to is in strictness to be regarded as 
lying somewhere (about midway) between these two 
boundaries. It defines the position of the intermediate 
point of zero amplitude in the N-wave profile where the 
velocity is sonic. 

Arrival. Initial record of a seismic wave on a seismograph 
recording, usually an abrupt change in form of recording line. 

Aspect Angle. The angle between the target plane and the 
common axis of the two microphones in an FEI transmitter, 
or the complement of the angle between the direction of the 
bullet trajectories near the FEI transmitter and the 
microphone axis. 

Aspect Angle Errors. Deviations either of sum or of differ¬ 
ence response which come from orientations of the micro¬ 
phone axis oblique to the target plane; that is to say, for 
aspect angles different from zero. 

AVC. Automatic volume control. 

Binary Counter. A simple electric counter consisting of two 
tubes which are made alternately conducting by suc¬ 
cessive impulses. 

Block I, Block III. Army-Navy identification of airborne 
television apparatus developed by Radio Corporation of 
America, Camden, New Jersey. 

CCI. An HPI based on change of color of a liquid. 

Channel (receiver channel). In the aperiodic FEI system, the 
shock-wave signals from each of the two microphones in 
the transmitter are separately transmitted at different radio 
carrier frequencies to the receiver where they are separately 
received in two different and distinct channels. The micro¬ 
phone signals remain in separate channels of the receiver 
up to the “sum tube.’’ 

Commutation System. The scanning of a number of chan¬ 
nels to reduce them to one electric channel for radio trans¬ 
mission. 

Condenser Microphone. A microphone in which a metal 
diaphragm forms one of the electrodes of an electric con¬ 
denser, the other (stationary) electrode being situated close 
to the back surface of this diaphragm. Minute diaphragm 
vibrations change the capacity of this condenser by chang¬ 
ing the air-gap spacing between these electrodes. 

CR. Check remover. 

Curie. A unit quantity of radium emanation or radon, defined 
as that quantity which is in equilibrium with 1 g Ra 3 . Its 
volume at NTP is about 0.63 mm. 

Decimal Counter. An electronic counter system for reporting 
in powers of ten. 

Delay Error. The error in miss indication coming from the 


fact that the shock wave reaches the FEI microphone at a 
later time than the instant when the bullet pierces the 
target plane and that, in consequence, the target will have 
moved to a new position. 

Density Pattern (of shots). See Normal or Gaussian Shot 
Density Pattern. 

Difference Lobes. The regions of the FEI acoustic pattern 
in the target plane inside of which the incidence of a shot 
causes the FEI to report the directionality of a miss. 

Difference Response. The difference response or direction¬ 
ality response in the definitive Model XI-A, FEI receiver 
is a signal appearing in that channel of the receiver corres¬ 
ponding to the one of the two microphones which receives 
the shock-wave signal first; it is proportional to the signal 
from that microphone alone. 

Difference-Response Lobes. See Lobes. 

Directionality (of miss). One of the functions of the FEI 
is to indicate the directionality of a miss, i.e., whether the 
miss passed fore or aft of the moving (towed) transmitter. 

Discriminator. An element of an f-m receiver which, over a 
limited range of input carrier frequencies, supplies a voltage 
V proportional in magnitude and sign to the deviation 
of the input frequency from a standard value. The char¬ 
acteristic, relating input frequency to output voltage, has 
the indicated shape. The frequency range A to B is the 


linear working range. 

/ i 

/ i . 

+ V 

B 


A 

-V 

1 

'FREQ-*' 


Doppler Effect (in acoustics). A change in the apparent 
period of an acoustic wave because of the motion of either 
the observer or the sound source relative to the medium. 

ECD. Electronic counter communication device. 

EG. High-speed counter auxiliary 1-megacycle electronic gate. 

Electronic Counter. An instrument which measures and 
records the number of electric pulses it receives from a 
suitably designed network. 

Electrostatic (Microphone) Tester. A device which per¬ 
mits oscillographic study of the relationship (in both 
amplitude and phase) between an input electrostatic driving 
force applied to a condenser microphone and the resulting 
instantaneous displacement of the microphone diaphragm. 
With it the natural frequency of a highly damped dia¬ 
phragm is determined as the frequency at which input force 
and displacement are in quadrature. Curves of response as 
functions of driving frequency can also be obtained with 
this device. 

EMU. Electromagnetic deflection unit. 

ER. Electroplated ribbon. 

ERT. Electron ratchet tube. 

FEC (Firing Error Camera). A camera utilizing the 
shock-wave signal from the FEI receiver to indicate by a 
photographic mark on the edge of motion picture film the 
instant when the shock wave from each bullet reaches the 
FEI transmitter in the target. The target and the tracer 
bullet are photographed on successive picture frames of the 
film so that measurements of these with appropriate correc¬ 
tion of the delay of th£ shock wave signal furnish data as 
to the magnitude of the miss for validation tests of the 
FEI in towed flight. 

FEI. Firing error indicator. 

FEO. Firing error oscillograph. 




201 




202 


GLOSSARY 


Firing Test, Static. See Static Firing Test. 

Flag Target. An airborne towed target which, for use with 
the FEI, must be made of plastic cloth in the shape of a 
banner or pennant. Velon is the usual plastic material. 

Gamma Ray. Electromagnetic radiation of very short wave 
length (of the order of 10 -8 mm) and of nuclear origin. 

Gauss, Law of. See Normal or Gaussian Shot Density Pattern. 

Geophone. A device for detecting or listening to sounds 
transmitted through the ground. 

Half-Life. Time required for the activity of a radioactive 
substance to decrease to half its initial value. 

Harmonic Mean Miss Distance. A gunner’s average miss 
distance determined by classifying his shots into radial 
miss-distance zones, dividing the number of shots in each 
zone by the mean radius of that zone, summing all these 
quotients, and dividing this sum by the total number of 
shots considered. The reciprocal of this result is the harmonic 
mean miss distance. 

HPI. Helium-purity indicator. 

HSDA. High-speed decimal accumulator. 

Hyorophone. An instrument for detecting or listening to 
sounds transmitted through the water. 

IF. Internal flux. 

Informing (function of the FEI). Continuously and imme¬ 
diately informing the gunner (or anyone else desired) of the 
qualitative nature of the errors of fire at the time they occur. 
It is distinguished from the scoring function which involves 
quantitative statistics regarding a large number of shots as to 
the radial miss-distance zones in which they fall. 

Limiter. A component used in f-m receivers to hold at a 
constant value the amplitude of the f-m signals. It consists 
of an r-f amplifier whose output-signal amplitude is inde¬ 
pendent of the input-signal amplitude, provided the latter 
stays above a certain critical (or “saturation”) voltage. 
Below saturation the limiter does not perform its function. 

Lobes (difference-response lobes or directionality lobes). Re¬ 
gions of the target plane on either side of the FEI transmitter 
such that the placement of a shot within one or the other 
will be registered in one or the other directional channel 
of the FEI receiver. 

LU. Light unit. 

Master Oscillator Power Amplifier (radio transmitter). 
This type of transmitter as used in the FEI system consists 
of a highly shielded frequency-determining oscillator whose 
tank capacity in part consists of the condenser microphone, 
so that it is frequency modulated by vibrations of the dia¬ 
phragm. A separate component, the power amplifier, fur¬ 
nishes power to the antenna (at the frequency controlled 
unilaterally by the master oscillator) so as to exert negligible 
reaction on the oscillator from changes in its load such as 
variations in antenna capacity, etc. 

Microphone Axis. In the aperiodic FEI, the axis through the 
centers of the two microphones, passing diametrically 
through the spherical transmitter. 

Miss Distance, Apparent. The distance from bullet tra¬ 
jectory to FEI transmitter at the instant when the latter 
receives the shock wave. 

Miss Distance, True. The distance measured from FEI 
transmitter to bullet at the instant when the bullet is in 
the target plane. 

MOPA. See Master Oscillator Power Amplifier. 

MRM. Magnetic recording media. 

MTR. Magnetic transient recorder., 

m/3. Feedback factor. 

Nitrostarch Equivalent. Amount of nitrostarch which must 
be exploded at point of impact to give same arrival mag¬ 
nitude (as bomb impact does) at detector. 


Normal or Gaussian Shot Density Pattern. The so-called 
law of Gauss for the distribution of shots aimed at a central 
point of a target. The simplest case is the one having circular 
symmetry in which the fraction of all shots placed in a 
zone between the radii r and r + dr is P(r)dr, where 

r 2 

P{r)dr = e ~ 2U* dr. 

In this law, R is called the gunner’s “most probable miss 
distance.” According to this law the number S of shots 
placed inside radius r is: 

r 8 

S{r) = 1 - e- 2 * 2 - 

N Wave (profile of ballistic shock wave). An acoustic wave 
form characteristic of ballistic shock waves consisting of an 
abrupt rise in pressure (the H discontinuity) followed by a 
linear pressure decline to a negative relative pressure and an 
abrupt return (the T discontinuity) to atmospheric pressure. 

ODG. Optical deflection unit. 

Parasitic (radio antenna). A supplementary antenna in which 
induced oscillations generate in conjunction with the main 
antenna a radiation pattern having desired directional 
sensitivity. 

Patterns (target). Lines of iso-sum response and also of iso¬ 
response of the directionality-indicating signal (in the re¬ 
ceiver) plotted to scale on the target plane so as to indicate the 
response zones and response lobes obtainable with the FEI. 

PCT. Powder-coated tape. 

Period (of N-shaped shock-wave profile). Period of time 
measured from the instant of transit of the head discontinu¬ 
ity H to the instant of transit of the tail discontinuity T 
past a point fixed in space relative to the air mass. 

PU. Power unit. 

Pulse Lengthening. An electronic method of increasing the 
duration of a brief transient electric pulse. A condenser 
of capacity C provided with a definite high-resistance shunt 
of resistance R (leak) is charged by a rectifying electronic 
element. The condenser thereafter discharges slowly with 
time constant RC starting with the peak value of the 
charging pulse and decaying exponentially. 

Quartz Piezoelectric Microphone. The microphone of this 
type used for the study of wave forms of shock waves con¬ 
sisted of a pair of quartz plates tightly enclosed in a metal 
box so that changes (both negative or positive) in the rela¬ 
tive external air pressure placed the plates under tension 
or compression. The resulting piezoelectric charges appear¬ 
ing on the metallized surfaces of the quartz plates were used, 
through suitable impedance transformers and amplifiers, to 
furnish on a single-sweep cathode-ray oscilloscope a record 
of the wave form which was photographically recorded. 

Radon. Radium emanation (atomic number 86). 

RBC. Resetting binary counter. 

Resonant FEI. An earlier form of FEI operating on one 
radio carrier frequency. Two separate channels of infor¬ 
mation were established between the FEI transmitter and 
receiver by the use of two microphones whose disphragms 
had different and highly sustained natural frequencies 
of vibration. 

Response Patterns. See Patterns. 

RH. Ring head. 

Round-to-Round Reproducibility. The variability of shock- 
wave amplitude received on different rounds under identical 
conditions as to caliber, miss distance, range from gun, 
and receiving apparatus. This variability is attributed to 
local atmospheric fluctuations of temperature, wind, veloc¬ 
ity, turbulence, etc. It is also in part a result of variations 
in bullet velocity though this effect is small. 



GLOSSARY 


203 


Scoring (function of the FEI). The rating of gunners as to 
excellence of markmanship on the basis of statistics regarding 
the per cent or fraction of all rounds of a given series which 
fall within specified FEI target zones. Scoring aims to 
classify gunners. 

Shock-Wave Discontinuities, H and T. See N Wave. 

Static Firing Test. Acoustic test of the response character¬ 
istics of FEI transmitters made with the transmitter sup¬ 
ported in a fixed position about 35 ft above the ground. 
Bullets of various calibers are shot at observed miss distances 
and positions in the target plane. The resulting response of 
the FEI transmitter is recorded with an FEI field measuring 
receiving station, or firing error oscilloscope. 

Stibitz (Dual) Photographic Theodolite. A method of 
photographing an aerial target and the tracer bullets fired 
at it, in which two theodolite motion picture cameras are 
used separated by a known base line, usually vertical. The 
method permits of determining the miss distance when the 
bullet has reached the target plane. 

Subcarrier System. A method of telemetering in the radio 
frequency range between 1 and 50 kc. 

Sum Response. The sum of the shock-wave signal amplitude 
from the two microphones in the FEI transmitter. 

Tank Circuit. The frequency-determining circuit of an oscil¬ 
lator consisting essentially of a capacity and an inductance 
in which oscillations occur at substantially the natural 
resonant frequency. 


Target Plane. A plane passing through the FEI transmitter 
perpendicular to the trajectories of bullets close thereto. 
Since the miss distances are small relative to the range 
from the gun, all bullet trajectories are nearly perpendicular 
to the target plane. 

Telemetering. The art, or practice, of transmitting signals 
(by radio or otherwise) indicative of quantitative measured 
values of one or more physical variables. 

Thermistor. Contraction of the words “thermal resistor” to 
describe a new type of circuit element whose resistance 
varies markedly with temperature. 

Unit-to-Unit Reproducibility. A measure of the repro¬ 
ducibility of successive FEI transmitter units as to their 
frequency shift in response to standard excitation of the 
microphone diaphragms. 

Vertical Filter. A filter which has a cutoff characteristic 
which goes from zero to infinite attentuation within an 
infinitesimal frequency change. 

VSI. An HPI based on velocity of sound in the helium-air 
mixture. 

Zones (sum response). Annular zones in the target plane 
lying essentially between circles concentric about the tran s- 
mitter. A shot placed inside a given such zone is so indicated 
at the receiver either on a tape recorder, a zone counter, 
or by informing lights. Within limits the radial boundaries 
of zones can be fixed by adjusting threshold bias potentials 
in the FEI receiver. 


Notation Frequently Used 


f m Frequency of the mth subcarrier. 

n Number of channels. 

A/ Pass-band of each channel of frequency selector. It is 
assumed that the pass-bands for all n channels are 
the same. 

F Sampling rate of commutation system. 

{ Frequency being sampled by commutator. 


F a Maximum possible audio frequency in output of fre¬ 
quency-modulated radio link. 

F r Maximum audio frequency required from radio link 
for commutation system. 

F h Maximum audio frequency required from link for sub¬ 
carrier system. 

- Fraction of allotted time each commutator channel is on. 

a 


( TTTl if yir.frUU 1 , A 1 










BIBLIOGRAPHY 


Numbers such as Div. 17-436.522-MI indicate that the document listed has been microfilmed and that its title 
appears in the microfilm index printed in a separate volume. For access to the index volume and to the microfilm, consult 
the Army or Navy agency listed on the reverse of the half-title page. 

The number of an OSRD report is specified in one of three ways in this bibilography. These are illustrated and 
explained as follows: (1) D3 77, meaning the number 77 was assigned to the report by Section D3 before the organization 
of Division 17 and before the uniform issuance of OSRD numbers for all OSRD reports; (2) 17.2-2, meaning the number 2 
was assigned to the report by Section 17.2 of Division 17; and (3) OSRD 1333, meaning the number 1333 was assigned 
to the report by OSRD under a uniform policy of assigning individual numbers to OSRD reports. 


Chapter 1 

National Broadcasting Company — -OEMsr-314 

1. The Application oj Television for Telemetering , F. J. 
Somers, Status Report 217, Research Projects PDRC- 
305 and RCA-173, NBC, Apr. 1, 1942. 

Div. 17-436.522-MI 

2. The Application of Television for Telemetering, F. J. 
Somers, Status Report 241, Research Projects PDRC-305 
and RCA-173, NBC, June 1, 1942. Div. 17-436.522-MI 

3. The Application of Television to Telemetering, (Final Report ), 

F. J. Somers, OSRD 1880, Research Project RCA-173, 
NBC, Apr. 30, 1943. Div. 17-436.522-M2 

Hazeltine Electronics Corporation — NDCrc-194 

4. Airplane Telemetering—Pickup Systems, J. Kelly Johnson, 

D3 64, NDCrc-194, Report 1215-W, Hazeltine, June 16, 
1941. Div. 17-436.51-MI 

5. Airplane Instrument Telemetering—Modulation Systems, 

J. Kelly Johnson, D3 66, Report 1217-W, Hazeltine, 
June 18, 1941. Div. 17-436.51-M2 

6. Progress Report: A irplane Instrument Telemetering, J. Kelly 
Johnson, D3 65, Hazeltine, Aug.4,1941. Div. 17-436.51-M3 

7. Airplane Instrument Telemetering—Progress Report No. 2, 
J. Kelly Johnson, D3 118, Hazeltine, Sept. 27, 1941. 

Div. 17-436.51-M3 

8. Airplane Instrument Telemetering—Progress Report No. 5, 
M. J. DiToro, D3 215, Hazeltine, Apr. 3, 1942. 

Div. 17-436.51-M3 

9. Airplane Instrument Telemetering—Progress Report No. 6, 
M. J. DiToro, D3 252, Hazeltine, July 8, 1942. 

Div. 17-436.51-M 3 

10. Telemetering Equipment Flight Tests at Wright Field., 

Dayton, Ohio, [Airplane Instrument Telemetering]— Pro¬ 
gress Report No. 7, M. J. DiToro, OSRD 1366, Hazeltine, 
Mar. 26, 1943. Div. 17-436.51-M4 

Rudolph Wurlitzer Company — OEMsr-247 

11. Confidential Report National Defense Research Committee 
Project — PDRC-267 for Proving Wurlitzer Radio Tele¬ 
metering, L. E. Hayslett, D3 295, Wurlitzer, July 2, 1942. 

Div. 17-436.511-M4 

12. Radio Telemetering of Aircraft Instruments—Pulse Method, 

L. E. Hayslett, OEMsr-247, OSRD 1459, Wurlitzer, 
Nov. 1, 1943. Div. 17-436.511-M5 

13. Radio Telemetering of Strain Gauge Indications (Final 

Report), L. E. Hayslett, OSRD 3214, Wurlitzer, Dec. 18, 
1944 . Div. 17-436.511-M6 

C. G. Conn, Ltd—OEMsr-1099 

14. Flight Test Recorder: Wattmeter Type Strain Gage Tele¬ 
metering Equipment, L. B. Greenleaf and E. L. Kent, 
OSRD 1945, Conn, Oct. 15, 1943. Div. 17-436.512-Ml 


15. Flight Test Recorder: Wattmeter Type Strain Gage Tele¬ 
metering Equipment, L. B. Greenleaf and E. L. Kent, 
OEMsr-1099, OSRD 3426, Conn, June 14, 1944. 

Div. 17-436.512-Ml 

Princeton University — OEMsr-1037 

16. Radio Telemetering of Strain Gauges by Electronic Com¬ 

mutation, M. W. Arsove, R. B. Blizard, J. F. Brinster, 
and others, OSRD 3385, Service Project NA-134, 
Princeton, Mar. 13, 1944. Div. 17-436.521-MI 

17. Radio Telemetering of Strain Gauges by Electronic Com¬ 
mutation (Supplement to OSRD Report No. 3385), B. 
Kurrelmeyer, W. H. Mais, E. H. Green, OSRD 3666, 
Service Project NA-134, Princeton, Mar. 30, 1944. 

Div. 17-436.521-MI 

18. Test of Electronic Telemetering Equipment at NAES July 1 

through July 10, M. W. Arsove, R. B. Blizard, J. F. 
Brinster, and others, OSRD 3936, Princeton, July 17, 
1944. ' Div. 17-436.521-M2 

19. Preliminary Flight Tests of Electronic Telemetering Equip¬ 

ment, August 8 through A ugust 22, M. W. Arsove, R. B. 
Blizard, J. F. Brinster, and others, OSRD 3936, Princeton, 
Sept. 2, 1944. Div. 17-436.521-M3 

20. Radio Telemetering of Strain Gauges by Electronic Com¬ 

mutation, It. B. Blizard, J. F. Brinster, D. B. Davis, and 
others, OSRD 4084, Service Project NA-134, Princeton, 

Dec. 6, 1944. Div. 17-436.521-M4 

21. Radio Telemetering of Flight Data, M. H. Nichols (co¬ 
ordinated by G. E. Beggs, Jr., Technical Aide, Section 
17.2, NDRC), OSRD 4448, Princeton, Feb. 28, 1945. 

Div. 17-436.5-MI 

21a. Ibid., pp. 97-99. 

21b. Ibid., p. 75. 

22. Bench Tests of NDRC Telemetering System, Type 1, Model B, 
W. W. McLean, OSRD 5041, Princeton, June 1, 1945. 

Div. 17-436.52-MI 

23. Radio Telemetering of Flight Data (Final Report), M. H. 
Nichols, OSRD 5679, Princeton, Oct. 22, 1945. 

Div. 17-436.5-M2 


24. Test Report No. 246, Aircraft Radio Laboratory, Wright 
Field, Dayton, Ohio, July 15, 1943—Sept. 1, 1943. 

25. “Load Rating for Multi-Channel Amplifiers,” B. C. 
Holbrook and J. T. Dixon, Bell System Technical Journal, 
Vol. XVI, No. 4, October 1937, p. 624. 

26. “Cross Modulation Requirements on Multi-Channel 
Amplifiers Below Overloading,” W. R. Bennett, Bell 
System Technical Journal, Vol. XIX, No. 4, October 1940, 
p. 587. 

27. “Time Division Multiplex Systems,” W. R. Bennett, 
Bell System Technical Journal, Vol. XX, No. 2, April 1941, 
p. 199. 



205 


206 


BIBLIOGRAPHY 


28. “The Magnetically Focused Radial-Beam Vacuum Tube,” 
A. M. Skellett, Bell System Technical Journal, Vol. 
XXIII, No. 2, April 1944, p. 190. 

29. “The Vultee Radio Recorder,” H. D. Giffen, Aeronautical 
Engineering Review, Vol. II, No. 7, July 1943, p. 9. 

30. “The Magnetically Focused Radial-Beam Vacuum Tube,” 
A. M. Skellett, Journal of Applied Physics, Vol. XV, 
No. 10, October 1944, p. 704. 

31. Preliminary Instructions for Radio Sonde Transmitting 
Equipment Model TDC and Radio Sonde Receiving Equip¬ 
ment Model RAU 2, Julien P. Friez and Sons, Division 
of Bendix Aviation Corporation. 

32. Private Communication from C. K. Stedman, Boeing 
Aircraft Company. 

Chapter 2 

California Institute of Technology — OEMsr-600 

1. Appendix I: The Geometry of Shock-Wave Response for the 
Spherical FEI Transmitter, Jesse W. M. DuMond and 
E. R. Cohen, (see footnote c, Section 2.1). 

Div. 17-443.34-M 4 

2. Appendix II: Sources of Error in the Aperiodic FEI, 

Jesse W. M. DuMond and E. R. Cohen, (see footnote c, 
Section 2.1). Div. 17-443.23-M3 

3. Appendix III: A Determination of the Wave-Forms and 

Laws of Propagation and Dissipation of Ballistic Shock- 
Waves, Jesse W. M. DuMond and E. R. Cohen, (see 
footnote c, Section 2.1). Div. 17-443.1-M4 

4. Appendix IV: Notes on Design and Technique of Manu¬ 

facture and Calibration of A periodic Condenser Microphones 
for the FEI, Jesse W. M. DuMond and E. R. Cohen, 
(see footnote c, Section 2.1). Div. 17-443.23-M4 

5. Appendix V: Auxiliary Standardizing and Measuring 
Equipment for the FEI, Jesse W. M. DuMond and E. R. 
Cohen, (see footnote c, Section 2.1). Div. 17-443-M3 

6. Progress Report on a Device Using Magnetized Bullets for 
Indicating Errors of Marksmanship in Training Anti- 
Aircraft Gun Crews, Alex E. S. Green and Wolfgang K. H. 
Panofsky, D3 268, CIT, Aug. 3, 1942. Div. 17-443-MI 

7. Supplementary Progress Report on the Magnetic Bullet 

Firing Error Indicator, Alex E. S. Green, Wolfgang K. H. 
Panofsky, D3 279, Service Projects AC-46, NO-173, and 
NO-260^ CIT, Sept, 2, 1942. Div. 17-443.21-MI 

8. Progress Report on the Development of Devices for I ndicating 

Errors in Marksmanship to Anti-Aircraft Gun Crews, 
Wolfgang K. H. Panofsky and Alex E. S. Green, D3 299, 
Service Projects AC-46, NO-173, and NO-260, CIT, 
Sept, 4, 1944. Div. 17-443-MI 

9. A pplication of Shock Wave to the Firing Indicator Problem, 

Wolfgang K. H. Panofsky and Alex E. S. Green, CIT, 
Sept. 16, 1942. ' Div. 17-443.1-MI 

10. Third Interim Bi-Monthly Progress Report on a Firing 

Error Indicator for Training Anti-Aircraft Gun Crews in 
Marksmanship on Moving Airborne Targets, Jesse W. M. 
DuMond, OSRD 1186, Service Projects AC-46, NO-173, 
and NO-260, CIT, Jan. 13, 1943. Div. 17-443-MI 

11. Book of Instructions, Electronic Marksmanship Scoring 
Device Ground Station, Models I and II, Wolfgang K. H. 
Panofsky, OSRD 1540, CIT, Mar. 18, 1943. 

Div. 17-443.35-MI 

12. Fourth Interim. Progress Report on the Development of 

Devices for Indicating Errors in Marksmanship on Airborne 
Practice Targets, Jesse W. M. DuMond, OSRD 1537, 
Service Projects AC-46, NO-173, and NO-260, CIT, 
Apr. 3, 1943. Div. 17-443-MI 

13. Experimental and Theoretical Study of Ballistic Shock 
Wave Amplitude as a Function of Range, Miss Distance, 


Caliber and Other Variables for A pplication to a Firing 
Error Indicator, Alex E. S. Green, Jesse W. M. DuMond, 
and Wolfgang K. H. Panofsky, OSRD 1646, CIT, 
June 31, 1943. Div. 17-443.1-M2 

14. Fifth Interim Progress Report on Firing Error Indicator, 
Jesse W. M. DuMond, OSRD 1647, Service Projects AC- 
46, NO-173, and NO-260, CIT, July 31, 1943. 

Div. 17-443-Ml 

15. Book of Instructions for the Acoustic Target Receiving 

Station (Model V), Wolfgang K. H. Panofsky, OSRD 1922, 
CIT, August 1943. Div. 17-443.33-MI 

16. The Tuned Microphone Bi-Directional Informing and Scor¬ 

ing System of the Acoustic Firing Error Indicator, Alex E. S. 
Green and Ronald R. Rau, OSRD 1648, CIT, Sept. 13, 
1943. Div. 17-443.34-Ml 

17. Sixth Interim Progress Report on Firing Error Indicator, 
Jesse W. M. DuMond, OSRD 3041, Service Projects 
AC-46, NO-173, and NO-260, CIT, Nov. 20, 1943. 

Div. 17-443-Ml 

18. Functional Specifications [for the] Acoustic Target Receiving 
Station, AN/GRR-1, CIT, Jan. 3, 1944. 

Div. 17-443.33-M2 

19. Seventh Interim Progress Report on Firing Error Indicator, 
Jesse W. M. DuMond, OSRD 3264, Service Projects 
AC-46, NO-173 and NO-260, CIT, Jan. 14, 1944. 

Div. 17-443-MI 

20. Eighth Interim Progress Report on Firing Error Indicator, 
Jesse W. M. DuMond, OSRD 3545, Service Projects, 
AC-46, NO-173 and NO-260, CIT, Mar. 18, 1944. 

Div. 17-443-MI 

21. Ninth Interim Progress Report on Firing Error Indicator, 
Jesse W. M. DuMond, OSRD 3951, CIT, June 23, 1944. 

Div. 17-443-Ml 

22. The Reliability of the Acoustic Firing Error Indicator, 
Jesse W. M. DuMond, OSRD 4966, Service Projects 
AC-46, NO-173, and NO-260, CIT, June 1944. 

Div. 17-443.22-Ml 

23. Tenth Interim Progress Report on Firing Error Indicator, 
Jesse W. M. DuMond, OSRD 4069, Service Projects 
AC-46, NO-173, and NO-260, CIT, Aug. 15, 1944. 

Div. 17-443-Ml 

24. Special informal Report on the Aperiodic Firing Error 

Indicator, CIT, October 1944. Div. 17-443.23-MI 

25. Appendices to Special Informed Report on the Aperiodic 
Firing Error Indicator, CIT, October 1944. 

Div. 17-443.23-MI 

26. Eleventh interim Progress Report on Firing Error indicator, 
Jesse W. M. DuMond, OSRD 4418, Service Projects, 
AC-46, NO-173, and NO-260, CIT, Nov. 1, 1944. 

26a. Ibid., pp. 16-19. Div. 17-443-Ml 

27. Twelfth Interim Progress Report on Firing Error Indicator, 
Jesse W. M. DuMond, OSRD 4467, Service Projects, 
AC-46, NO-173, and NO-260, CIT, Dec. 7, 1944. 

Div. 17-443-Ml 

28. Thirteenth Interim Progress Report on Firing Error Indi¬ 
cator, Jesse W. M. DuMond, OSRD 4664, Service Projects, 
AC-46, NO-173, and NO-260, CIT, Jan. 29, 1945. 

Div. 17-443-Ml 

29. The Aperiodic Firing Error Indicator, Wolfgang K. H. 
Panofsky, OSRD 4967, CIT, May 1945. 

Div. 17-443.23-M2 

30. Appendices to the Aperiodic Firing Error Indicator, Wolf¬ 
gang K. H. Panofsky, OSRD 4968, CIT, May 1945. 

Div. 17-443.23-M2 

30a. Ibid., Appendix I. 

30b. Ibid., Appendix VII. 

31. Fourteenth Interim Progress Report on Firing Error Indi¬ 
cator, Jesse W. M. DuMond and E. R. Cohen, OSRD 





BIBLIOGRAPHY 


207 


5260, Service Projects, AC-46, NO-173, and NO-260, 
CIT, July 1, 1945. Div. 17-443-MI 

32. Flight Validation of the Firing Error Indicator, D. G. 

Marlow, W. E. Deeds, and E. R. Cohen, OSRD 5553, 
Service Projects, AC-46, NO-173, and NO-260, CIT, 
September 1945. Div. 17-443.4-MI 

33. Functional Specifications Firing Error Indicator System, 
Research Project 17.3-13, CIT, Sept. 11, 1945. 

Div. 17-443.34-M2 

34. Final Report on the Acoustic Firing Error Indicator, 
Jesse W. M. DuMond, OSRD 5733, Service Projects 
AC-46, NO-173, and NO-260, CIT, Sept, 15, 1945. 

Div. 17-443.22-M2 

35. Instruction Book for Firing Error Camera, CIT, September 

1945. Div. 17-443.32-MI 

36. Book of Instructions, Firing Error Oscillograph, Research 
Project 17.3-13, CIT, Oct. 1, 1945. Div. 17-443.31-MI 

37. Book of Instruction {Preliminary ), Firing Error Indicator 

Receiving Station M XI-A, Research Project 17.3-13, 
CIT, October 1945. Div. 17-443.33-M3 

38. Operating Instructions and Functional Specifications for 

Model D Field Test Instrument, Research Project 17.3-13, 
CIT, Oct. 31, 1945. Div. 17-443.35-M2 

Hoffman Radio Corporation — OEMsr-1108 

39. Final Report on Firing Error Indicator: Instruction Book 

for the Firing Error Indicating Equipment, Hoffman, 
Feb. 1, 1945. Div. 17-443-M2 

Western Electric Company — OEMsr-1457 

40. Condenser Transmitter Development, W. A. Munson, 

D. W. Farnsworth, and S. Balashek, OSRD 6398, WEC, 
Sept, 15, 1945. Div. 17-443.34-M3 


41. A Personal Evaluation of the Firing Error Indicator Project, 
L. J. Sivian, NDRC, Oct. 4, 1945. Div. 17-443.1-M3 

42. “A High Speed Mechanical Recorder,” H. Victor Neher, 
Renew of Scientific Instruments, Vol. 10, No. 1, January 
1939, p. 29. 

43. “On Shock Waves,” L. D. Landau, Journal of Physics 
(Academy of Sciences of the USSR), Vol. 6, No. 5, 1942, 
pp. 229-230. 

44. “Stosselwelle und Detonation,” R. Becker, Zeitschrift fur 
Physik, Vol. 8, 1921, p. 321. 

45. Time Bases, Owen Standige Puckle, John Chapman and 
Sons, London, 1942. 

Chapter 3 

Brush Development Company — OEMsr-264 

1. First Report on the Development of a High Frequency Strain 
Analyzer, OSRD 1615, D3 212, Brush. Div. 17-510-MI 

2. Second Report on the Development of a High Frequency 
Strain Analyzer, OSRD 1615, D3 256, Brush. 

Div. 17-510-MI 

3. Third Report on the Development of a High Frequency Strain 
Analyzer, OSRD 1615, D3 305, Brush. Div. 17-510-Ml 

4. Summarized Information on the High Frequency Transient 

Recorder, Brush, [Nov. 25, 1942]. Div. 17-500-MI 

5. Fourth Report on the Development of a High Frequency St rain 
Analyzer, OSRD 1615, Brush, June 1, 1943. 

Div. 17-510-Ml 

6. Fifth Report on the Development of a High Frequency Strain 
Analyzer, OSRD 1615, Brush, July 27, 1943. 

Div. 17-510-Ml 

7. Report on the Magnetic Disc Plating Derived Under Con¬ 
tract No. OEMsr- 254, OSRD 1946, Brush, Dec. 1, 1943. 

Div. 17-500-M2 


8. Investigations on New Magnetic Recording Media, M. D. 
Temple, L. O. Olsen, E. C. Crittenden, Jr., and others, 
OSRD 3399, Brush, Feb. 29, 1944. Div. 17-500-M3 

9. Investigations on New Magnetic Recording Media (Final), 
O. Kornei, M. D. Temple, P. P. Zapponi, and others, 
OSRD 5325, Brush, June 30, 1945. Div. 17-500-M4 

***** 

10. “A Magnetic Recorder,” T. J. Malloy, Electronics, Vol. 11, 
No. 1, January 1938, p. 30. 

11. “Magnetic Recording,” S. J. Begun, Electronics, Vol. 11, 
No. 9, September 1938, p. 30. 

12. “Magnetic Recording and Some of Its Applications in the 
Broadcast Field,” S. J. Begun, Proceedings of the Institute 
of Radio Engineers, Vol. 29, No. 8, August 1941, p. 423. 

13. “A New Instrument for RecordingTransient Phenomena,” 

S. J. Begun, American Institute of Electrical Engineers 
Technical Paper, 42-57, December 1941. 

14. “Investigation of Magnetic Tape Recorders,” M. C. 
Selby, Electronics, Vol. 17, No. 5, May 1944, p. 133. 

15. “The Mechanism of Supersonic Frequencies as Applied 
to Magnetic Recording,” H. Toomim and D. Wildfeuer, 
Proceedings of the Institute of Radio Engineers, Vol. 32, 
No. 11, November 1944, p. 664. 

16. “Techniques for Evaporation of Metals,” R. O. Olsen, 
C. S. Smith, and E. C. Crittenden, Journal of Applied 
Physics, Vol. XVI, No. 7, July 1945, p. 425. 

17. “Supersonic Bias for Magnetic Recording,” L. C. Holmes 
and D. L. Clark, Electronics, Vol. 18, No. 7, July 1945, 

p. 126. 

18. “Frequency-Modulated Magnetic-Tape Transient Re¬ 
corder,” H. B. Shaper, Proceedings of the Institute of Radio 
Engineers, Vol. 33, No. 11, November 1945, p. 753. 

Chapter 4 

Hathaway Instrument Company — OEMsr-823 

1. Development of a Special Multi-Element Oscillograph, 

Claude M. Hathaway, OSRD 1688, Hathaway, July 19, 
1943. * Div. 17-323.62-MI 

2. Special Oscillograph for Aberdeen Proving Ground, Herbert 

Reno and Claude M. Hathaway, OSRD 3321, Hathaway, 
Jan. 15, 1944. Div. 17-323.62-M2 

3. Special Oscillograph for Aberdeen Proving Ground, Herbert 

Reno and Claude M. Hathaway, OSRD 4344, Hathaway, 
Aug. 31, 1944. ” Div. 17-323.62-M2 

Purdue Unicersity — OEMsr-920 

4. Report on Cathode Ray Oscillograph, H. J. Heim and 
Richard C. Webb, OSRD 1906, Purdue, Nov. 4, 1943. 

Div. 17-323.61-MI 

5. Report on Cathode Ray Oscillograph, H. J. Heim and 
Richard C. Webb, OSRD 3216, Purdue, Mar. 1, 1944. 

Div. 17-323.61-MI 

6. Report on Four Unit Cathode Ray Oscillograph, H. J. Heim 
and Richard C. Webb, OSRD 4937, Purdue. June 15, 1945. 

Div. 17-323.61-M3 

7. Handbook of Instructions for Operation and Maintenance — 
Four Unit Cathode Ray Oscillograph, H. J. Heim and 
Richard C. Webb, Purdue, June 15, 1945. 

Div. 17-323.61-M4 

White Research Associates — OEMsr-1211 

8. The Design and Construction of a Multi-Channel Recording 
Cathode Ray Oscillograph, C. B. White, R. H. McFee, 
and others, OSRD 3322, White Research, June 1, 1944. 

Div. 17-323.61-M2 




208 


BIBLIOGRAPHY 


9. The Design and Construction of a M ulti-Channel Recording 
Cathode Ray Oscillograph (Final Report), OSRD 4520, 
White Research, Oct. 30, 1945. 

Shell Oil Company, Inc. — OEMsr-1308 

10. Army Air Force Instrument Trailer (Final Report ), Shell, 
Oct. 18, 1944. Div. 17-323.1-MI 


11. “A-C Operated D-C Amplifier,” S. E. Miller, Electronics, 
Vol. XIV, No. 11, November 1941, p. 27. 

Chapter 5 

University of Illinois — OEMsr-2J+l 

1. Progress Report No. 1, May 1, [Three to Twenty Million 

Volt Radiography], D. W. Kerst, D3 214, U. Illinois, 
May 1, 1942. ' Div. 17-323.7-M3 

2. Progress Report No. 2, July 1, [Three to Twenty Million 

Volt Radiography], D. W. Kerst, D3 243, U. Illinois, 
July 1, 1942. ‘ Div. 17-323.7-M3 

3. Final Report on 3 to 20 Million Volt Radiography, D. W. 
Kerst, D3 278, U. Illinois, Sept. 1, 1942. Div. 17-323.7-M3 

4. First Bi-Monthly Report on 3 to 20 M V Radiographic Work, 
D. W. Kerst, D3 314, U. Illinois, Nov. 1, 1942. 

Div. 17-323.7-M3 

5. Second Report on 3 to 20 Million Volt Radiography Con¬ 
tract, D. W. Kerst, Report 17.2-2, U. Illinois, Jan. 1, 1943. 

Div. 17-323.7-M3 

6. Part I: Determination of Characteristic Curves, G. D. 
Adams; Part II: Recent Developments of the Betatron, D. W. 
Kerst, OSRD 1333, U. Illinois, Mar. 1, 1943. 

7. Report on 3 to 20 Million Volt Radiography, D. W. Kerst, 
Report 17.2-6, U. Illinois, May 1, 1943. Div. 17-323.7-M3 

8. Report on 3 to 20 Million Volt Radiography, D. W. Kerst, 
OSRD 1566, U. Illinois, June 26, 1943. Div. 17-323.7-M3 

9. Part I: Development of New 20 MEV Betatron and of 

Porcelain Vacuum Tube; Part II: Assembly and Operation 
of the f.o MEV Betatron, D. W. Kerst, G. M. Almy, 
G. D. Adams, and H. W. Koch, OSRD 1944, U. Illinois, 
Sept. 1, 1943. Div. 17-323.7-M4 

10. Three to Twenty Million Volt Radiography — Progress 
Report, G. M. Almy, G. D. Adams, and R. K. Hursh, 
OSRD 3075, U. Illinois, Jan. 1, 1944. Div. 17-323.7-M3 

11. Three to Twenty Million Volt Radiography—Progress 
Report, G. M. Almy, G. D. Adams, and R. K. Hursh, 
OSRD 3384, U. Illinois, Mar. 1, 1944. Div. 17-323.7-M3 

12. Three to Twenty Million Volt Radiography—Progress 
Report, G. M. Almy, G. D. Adams, and R. K. Hursh, 
OSRD 3667, U. Illinois, June 1, 1944. Div. 17-323.7-M5 

13. Summary Report on 20,000,000 Volt Betatron, G. M. Almy 
and G. D. Adams, OSRD 3668, U. Illinois, June 1, 1944. 

Div. 17-323.7-M6 

14. Three to Twenty Million Volt Radiography—Progress 
Report, G. M. Almy, G. D. Adams, and R. K. Hursh, 
OSRD 4210, U. Illinois, Sept, 1, 1944. Div. 17-323.7-M5 

15. Description and Operating Instructions of a 3.5 MEV 

Betatron, Model 43, H. W. Koch and G. M. Almy, 
U. Illinois, Sept, 1, 1944. Div. 17-323.7-M8 

16. Three to Twenty Million Volt Radiography—Progress 
Report, G. M. Almy, G. D. Adams, and R. K. Hursh, 
OSRD 4613, U. Illinois, Jan. 1, 1945. Div. 17-323.7-M3 

17. The Application of the Betatron to Practical Radiography, 

G. M. Almy and G. D. Adams, OSRD 4883, U. Illinois, 
May 1, 1945. Div. 17-323.7-M9 

18. Three to Twenty Million Volt Radiography (Final Report), 

G. M. Almy and G. D. Adams, OSRD 5067, U. Illinois, 
June 30, 1945. Div. 17-323.7-M13 


Allis-Chalmers Manufacturing Company — OEMsr-1153 
None 

Massachusetts Institute of Technology — OEMsr-294 

19. First Bi-Monthly Report to the NDRC on High Voltage 

Radiography Research at MIT, R, J. Van de Graaff, 
D3 167, MIT, Jan. 1, 1942. Div. 17-323.7-M2 

20. On the M.I.T. Project in High Voltage Radiography, MIT, 

Nov. 1, 1941 to Jan. 15, 1943. Div. 17-323.7-MI 

21. On the M.I.T. Project in High Voltage Radiography, 
OSRD 1495, MIT, Jan. 15, 1943 to June 15, 1943. 

Div. 17-323.7-MI 

22. A Summary Report on the Application of High Voltage 
Electrostatic X-Ray Generators to Radiography, E. A. 
Burrill, Jr., OSRD 3677, MIT, June 29, 1944. 

Div. 17-323.7-M7 

23. Final Report on the M.I.T. Project in High Voltage 
Radiography: 

Volume 1: Brief Introduction and Summary; 

Volume 2: Two-Million-Volt Electrostatic X-Ray Generator; 
Volume 3: Design and Developmental Research in Connec¬ 
tion with the High Voltage Electrostatic Gener¬ 
ator; 

Volume 4: Production, Absorption, and Scattering of High 
Voltage X-Rays, OSRD 4488, MIT, June 1, 
1945. Div. 17-323.7-M10 

Volume 5: High Voltage Radiographic Techniques and 
Accessories; 

Volume 6: Photographic Aspects of High Voltage Radiog¬ 
raphy, OSRD 4488, MIT, June 1, 1945. 

Div. 17-323.7-M 11 

Volume 7: Detailed Drawings of X-Ray Generator Parts, 
OSRD 4488, MIT, June 1, 1945. 

Div. 17-323.7-M 12 

Chapter 6 

Gulf Research and Deielopment Company—OE Mar-266 

1. Telemetering Retardation Meter for Bombs, OSRD 1322, 

Gulf Research, Feb. 1, 1943. Div. 17-323.2-MI 

2. Telemetering Retardation Meter for Bombs, OSRD 1612, 

Gulf Research, Apr. 1, 1943. Div. 17-323.2-Ml 

3. Telemetering Retardation Meter for Bombs, OSRD 1556, 

Gulf Research, June 1, 1943. ' Div. 17-323.2-Ml 

4. Bomb Instrumentation—Telemetering Retardation Meter 
for Bombs, L. J. Peters, Thomas Bardeen, and E. J. Krack, 
OSRD 1888, Gulf Research, Aug. 1,1943. Div. 17-323.2-M2 

5. Bomb Instrumentation — Seismic Measurements of Bomb 
Impact, L. J. Peters, Thomas Bardeen, and E. J. Krack, 
OSRD 2000, Gulf Research, Sept. 1,1943. Div. 17-323.3-MI 

6. Bomb Instrumentation—Telemetering Retardation Meter for 
Bombs, L. J. Peters, Thomas Bardeen, and E. J. Krack, 
OSRD 3077, Gulf Research, Oct. 1,1943. Div. 17-323.2-M2 

7. Bomb Instrumentation—Telemetering Retardation Meter for 
Bombs ( Final Report), L. J. Peters, Thomas Bardeen, and 
E. J. Krack, OSRD 3704, Gulf Research, May 10, 1944. 

Div. 17-323.2-M3 

8. Bomb Instrumentation—Seismic Measurements of Bomb 
Impact, L. J. Peters, Thomas Bardeen, and E. J. Krack, 
OSRD3680, Gulf Research, May 15,1944. Div. 17-323.3-MI 

Chapter 7 

Faximile, Inc., — OEMsr-1203 

1. Final Report — OSRD 4566 (Including Instruction Manual 
Electromagnetic Deflection Unit Types EMU-3 and EMU- 
3B), Faximile, Nov. 20, 1944. Div. 17-323.52-MI 





BIBLIOGRAPHY 


209 


2. Final Report — OSRD 4-871 (Including Instruction Manual 
Optical Deflection Gauge Type A), Faximile, May 31, 
1945. Div. 17-323.52-M2 

Chapter 8 

The Texas Company — OEMsr-1369 

1. Measurements on Propeller Blades, Gerhard Herzog, 
OSRD 4957, Texas Company, Dec. 16, 1944. 

Div. 17-323.53-MI 

Chapter 9 

National Cash Register Company — NDCrc-63 {1-7 ); 
OEMsr-274 {8-13) 

1. Progress Report No. 1, High Speed Electronic Accumulator 
Research, Joseph R. Desch, D3 4, NCR, Jan. 27, 1941. 

2. Progress Report No. 2, High Speed Electronic Accumulator 
Research, Joseph R. Desch, D3 13, NCR, Mar. 31, 1941. 

Div. 17-436.2-M2 

3. Progress Report No. 3, High Speed Electronic Accumulator 
Research, Joseph R. Desch, D3 44, NCR, May 29, 1941. 

Div. 17-436.2-M2 

4. Progress Report No. 4, High Speed Electronic Accumulator 

Research, Joseph R. Desch, D3 88, NCR, Aug. 1, 1941, 
Figure 13. Div. 17-436.2-M2 

5. Progress Report No. 5, High Speed Electronic Accumulator 
Research, Joseph R. Desch, D3 121, NCR, Oct. 1, 1941. 

Div. 17-436.2-M2 

6. Progress Report No. 6, High Speed Electronic Accumulator 
Research, Joseph R. Desch, D3 166, NCR, Jan. 6, 1942. 

Div. 17-436.2-M2 

7. Final Report of Work Accomplished under Contract NDCrc- 
63 and Supplement No. 1 to Contract NDCrc-63. [Work on 
Ultra-High-Speed Decimal Accumulator], December 1, 1940 
to January 31, 1942, Joseph R. Desch, D3 199, NCR, 
Mar. 17, 1942, pp. 14-21. 

7a. Ibid., Figure 6. 

7b. Ibid., pp. 10-14. 

7c. Ibid., pp. 21-32. 

7d. Ibid., Pdgure 9. 

7e. Ibid., pp. 2, 3. 

7f. Ibid., pp. 2, 4. 

7g. Ibid., pp. 2, 4-6. 

7h. Ibid., pp. 2, 6. 

7i. Ibid., pp. 2, 6, 7. 

7j. Ibid., pp. 2, 9. 

8 . Progress Report No. 7, Communication Research Involving 

Impulse Counting. Work accomplished under Contract No. 
OEMsr-274, Joseph R. Desch, D3 205, NCR, Mar. 19, 
1942. Div. 17-436.2-M3 

9. Progress Report No. 8, Communication Research Involving 

Impulse Counters. Work accomplished under Contract No. 
OEMsr-274, Joseph R. Desch, D3 233, NCR, May 15, 
1942. Div. 17-436.2-M3 

10. One Megacycle Impulse Counter and Controlled Impulse 
Generator, NCR, July 14, 1942. 

11 . Progress Report No. 9 Communication Research Involving 

Impulse Counters. Work accomplished under Contract No. 
OEMsr-274■ Final Report, Joseph R. Desch, OSRD 1190, 
NCR, Oct. 1, 1942. Div. 17-436.2-M4 

12. Special Report. Application of High Speed Counter to Time 

Interval Measurements, E. Vincent Gulden, D3 327, 
NCR, Nov. 20, 1942. Div. 17-436.2-M5 

13. Identification System Using High-Speed Electronic Counter 
Circuit, Frank X. Bucher, D3 326, NCR, Nov. 28, 1942. 

Div. 17-436.2-M6 


University of Chicago 
NDCrc-68 {14,15 ); OEMsr-125 {16,17) 

14. First Progress Report on Counters, Volney C. Wilson, 
D3 41, U. Chicago, May 22, 1941. Div. 17-436.2-MI 
14a. Ibid., pp. 1-4. 

15. Final Report on Investigation Carried out between January 1 
and June 30,1941 under Contract No. NDCrc-68, [Counters], 
Volney C. Wilson, D3 120, U. Chicago, Sept. 30, 1941. 

Div. 17-436.2-Ml 

16. Progress Report on an Investigation Carried out between 
August 1 and October 1, 1941, under Contract No. OEMsr- 
125, R. J. Moon, D3 122, U. Chicago, Nov. 25, 1941. 

Div. 17-436.2-Ml 

17. Progress Report on an Investigation Carried out between 

October 1, 1941 and March 31, 1942, under Contract 
No. OEMsr-125, R. J. Moon, D3 203, U. Chicago, 
Apr. 1, 1942. Div. 17-436.2-Ml 

Massachusetts Institute of Technology — NDCrc-146 

18. Report D3 5, MIT, Feb. 8, 1941. 

19. Digitrons and Counting Devices, W. P. Overbeck, D3 25, 
MIT, Apr. 15, 1941. 

20. Digitrons and Counting Devices, D3 47, MIT, June 1, 1941. 

21. Digitrons and Counting Devices, W. P. Overbeck, C. W. 
Mueller, and P. F. Mooney, D3 96, MIT, Aug. 15, 1941. 

22. The Digitron, W. P. Overbeck, D3 98, MIT, Aug. 20, 1941. 


23. Electronic Counters, George R. Harrison and John S. 
Coleman, OSRD 58, Section D3, NDRC, Nov. 4, 1941. 

24. Special Report on an Electronic Counter Communication 
System, Section D3, NDRC, Sept. 29, 1942. 

Chapter 10 

Johns Hopkins University — OEMsr-1241 

1. Development of Methods for Determining the Proper Seating 
of Gilding Metal Rotating Bands on 105 MM. Shells in 
Production, W. B. Kouwenhoven, OSRD 3627, Service 
Project OD-151 Johns Hopkins, May 25, 1944, p. 20. 

Div. 17-610-Ml 

la. Ibid., p. 19. 

lb. Ibid., p. 15. 

lc. Ibid., pp. 23, 25. 

l d. Ibid., p. 14. 

le. Ibid., p. 10. 

l f. Ibid., p. 5. 

lg. Ibid., p. 16. 

2. Development of Methods for Determining the Proper Sealing 

of Gilding Metal Rotating Bands on 105 MM. Shells in 
Production, W. B. Kouwenhoven, OSRD 4576, Service 
Project OD-151 Johns Hopkins, Jan. 13, 1945, p. 7 and 
Appendix C. Div. 17-610-Ml 

3. Development of Methods for Determining the Proper Seating 
of Gilding Metal Rotating Bands on 105 MM. Shells in 
Production {Final Report), W. B. Kouwenhoven, OSRD 
4869, Johns Hopkins, Mar. 31, 1945, p. 7. Div. 17-610-M2 
3a. Ibid., Section I, p. 12. 

3b. Ibid., Section II, p. 16. 

3c. Ibid., Section III, p. 20. 

Chapter 11 

University of Pennsylvania — NDCrc-189 

1. Report on Helium Project, Gaylord P. Harnwell, D3 77, 

U. Penn, July 2, 1941. ‘ Div. 17-323.83-MI 

2. Report on Helium Project, Gaylord P. Harnwell, D3 113, 

U. Penn, Sept. 10, 1941. ' Div. 17-323.83-MI 



210 


BIBLIOGRAPHY 


3. Report on Helium Project, Gaylord P. Harnwell, D3 139, 

U. Penn, Nov. 10, 1941. ' Div. 17-323.83-MI 

Gulf Research and Development Company — OEMsr-266 

4. Development of Improved Helium Purity Indicator, D3 345, 
Gulf Research, Sept. 1-Nov. 1, 1942. Div. 17-323.83-M2 

Chapter 12 

Western Electric Company — OEMsr-868 

1. Battle Noise Equipment, E. M. Honan, OSRD 3109, 
Research Project 17.3-6, WEC, Jan. 31, 1944. 

Div. 17-411.1-MI 

2. Battle Noise Equipment, E. M. Honan, OSRD 4595, 
WEC, Jan. 16, 1945. 

Chapter 13 

Western Electric Company — OEMsr-498 

1. Energy Distribution in Machine Gun Sounds, J. P. Max- 

field and N. G. Wade, OSRD 1727, Service Project SC-27, 
WEC, July 21, 1943. Div. 17-422-MI 

2. The Character of Sounds from Army Vehicles, F. K. 
Harvey, G. F. Hull, Jr., R. T. Jenkins, and others, OSRD 
4254, Service Project SC-27, WEC, Aug. 21, 1944. 

Div. 17-421-MI 

3. The Analysis of Sounds from Mortars, F. K. Harvey, 
G. F. Hull, Jr., R. T. Jenkins, and others, OSRD 4393, 
Service Project SC-27, WEC, Nov. 15, 1944. 

Div. 17-422-M2 

4. Volume I — Analysis of Sounds from Field Artillery and 
Machine Guns; Volume II—Atlas of Oscillograms of Sounds 
from Field Artillery and Machine Guns, G. F. Hull, Jr., 
R. T. Jenkins, J. B. Kelly and N. G. Wade, OSRD 4594, 
Service Project SC-27, WEC, Jan. 15, 1945. 

Div. 17-422-M3 

Chapter 14 

Carnegie Institute of Technology — NDCrc-84 

1. Thermistor Investigation Report No. 1, J. F. Lamb and 
B. R. Teare, Jr., D3 23, Carnegie Tech, Apr. 8, 1941. 

2. Self-Balancing and Phase Shifting Bridge Circuits, J. F. 
Lamb and B. R. Teare, Jr., D3 51, Carnegie Tech, 
June 1, 1941. 

The Franklin Institute — NDCrc-55 

3. Trigger Amplifiers, T. H. Johnson, D3 6, Franklin Insti- 
ture, Feb. 28, 1941. 

4. Thermistor Trigger Circuits, T. H. Johnson, D3 17, Frank¬ 
lin Institute, Apr. 21, 1941. 

5. Trigger Amplifiers, T. H. Johnson, M. A. Pomerantz, and 
W. C. Sheppard, D3 50, Franklin Institute, June 26, 1941. 

Gulf Research and Development Company 
NDCrc-97 {6-10); OEMsr-266 {11) 

6. Development of Telemetric Flow Meter for A utomotive Fuels 
Embodying the Use of Thermistors, E. M. Palmer, D3 21, 
Gulf Research, Apr. 1, 1941. 

7. Development of Telemetric Flow Meter for A utomotive Fuels 
Embodying the Use of Thermistors, E. M. Palmer, D3 55, 
Gulf Research, June 1, 1941. 

8. Development of Telemetric Flow Meter for A utomotive Fuels 
Embodying the Use of Thermistors, E. M. Palmer, D3 90, 
Gulf Research, Aug. 1, 1941. 

9. Development of Telemetric Flow Meter for A utomotive Fuels 
Embodying the Use of Thermistors, E. M. Palmer, D3 254, 


Gulf Research, Dec. 31, 1941. 

10. Investigation of Telemetric Flow Meter for A utomotive F uels, 
E. M. Palmer, D3 255, Gulf Research, June 30, 1942. 

11. Development of Electric Frequency Meter, or Tachometer, 
L. L. Nettleton, D3 264, Gulf Research, Aug. 10, 1942. 

Div. 17-323.82-MI 

Harvard University 

NDCrc-58 {12-14); OEMsr-60 {15-19) 

12. High Speed Thermistors, Roger W. Hickman, D3 22, 
Harvard, Apr. 5, 1941. 

13. High Speed Thermistors, Roger W. Hickman, D3 58, 
Harvard, June 10, 1941. 

14. High-Speed Thermistors, Roger W. Hickman, D3 97, 
Harvard, Aug. 15, 1941. 

15. Thermistor Investigations, Roger W. Hickman, D3 134, 
Harvard, Nov. 1, 1941. 

16. Thermistor Investigations, Roger W. Hickman, D3 158, 

Harvard, Dec. 29, 1941. Div. 17-451-M7 

17. Thermistor Investigations, Roger W. Hickman, D3 194, 

Harvard, Feb. 15, 1942. Div. 17-451-M7 

18. Thermistor Investigations , D3 242, Harvard, June 6, 1942. 

Div. 17-451-M7 

19. Thermistor Investigations {Final Progress Report), D3 272, 

Harvard, Aug. 7, 1942. Div. 17-451-M8 

Massachusetts Institute of Technology 
NDCrc-142 {20,21); OEMsr-155 {22-27); NDCrc-179 {28-30) 

20. Properties of Thermistor Materials, A. B. White and W. B. 
Nottingham, D3 37, MIT, May 1, 1941. 

21. Properties of Thermistor Materials II, A. B. White and 
W. B. Nottingham, D3 73, MIT, July 1, 1941. 

22. Progress Report Contract No. OEMsr-155, A. V. deForest, 
D3 151, MIT, Dec. 13, 1941. 

23. Progress Report Contract No. OEMsr-155, A. V. deForest, 
D3 175, MIT, Jan. 31, 1942. 

24. Firing Strains in a 37-mm Field Gun, A. V. deForest, 

D3 183, MIT, Feb. 16, 1942. Div. 17-453-Ml 

25. Report on Contract No. OEMsr-155, A. V. deForest, 
D3 204, MIT, Apr. 3, 1942. 

26. Final Progress Report on Contract No. OEMsr-155, A. V. 
deForest, D3 287, MIT, Aug. 31, 1942. 

27. High-Speed Strain Transient Recording and Measurement 
Internal Gun Diameter under High Hydrostatic Pressure, 
A. V. deForest, A. C. Ruge, and G. S. Burr, D3 339, 
MIT, Dec. 1, 1942. 

28. Progress Report Project No. NDCrc-179, R. D. Evans, 
D3 63, MIT, June 15, 1941. 

29. Progress Report Project No. NDCrc-179, R. D. Evans, 
D3 110, MIT, Sept, 1, 1941. 

30. Development of an Instrument Capable of Measuring the 
Radon Content of Breath and Room Air Samples, R. D. 
Evans, S. C. Brown, and L. G. Elliott, D3 136, MIT, 
Oct. 31, 1941. 

Northwestern University — NDCrc-62 

31. Concerning the Thermistor Bolometer, Noel C. Jamison, 

D3 35, Northwestern, Apr. 30, 1941. Div. 17-451-M4 

32. The Thermistor Bolometer, Noel C. Jamison, D3 80, 

Northwestern, July 9, 1941. Div. 17-451-M5 

Rensselaer Polytechnic Institute — NDCrc-54 

33. Progress Report on Thermistor Investigation, D3 26, RPI, 

Apr. 15, 1941. Div. 17-451-M3 

34. Report on Thermistor Investigation, D3 85, RPI, July 31, 

1941. Div. 17-451-M3 


jTiiiTmimrn i 



BIBLIOGRAPHY 


211 


University of Pennsylvania 
NDCrc-102 {35-37); OEMsr-llO {38-41) 

35. Report No. 1 on Strain Gauge Investigation, Gaylord P. 
Harnwell, D3 32, U. Penn, Apr. 25, 1941. 

Div. 17-436.511-MI 

36. Report No. 2 on Strain Gauge Investigation, Gaylord P. 
Harnwell, D3 59, U. Penn, June 10, 1941. 

Div. 17-436.511-MI 

37. Final Report on Strain Gauge Investigation, Gaylord P. 
Harnwell, D3 94, U. Penn, Aug. 10, 1941. 

Div. 17-436.511-M2 

38. Report on Strain Gauge Investigation, Gaylord P. Harnwell, 
D3 135, U. Penn, Nov. 1, 1941. 

39. Report on Strain Gauge Investigation, Gaylord P. Harnwell, 
D3 174, U. Penn, Jan. 15, 1942. 

40. Progress Report on Contract No. OEMsr-110, [Strain Gauge 

Demonstration], Gaylord P. Harnwell, D3 206, U. Penn, 
Apr. 6, 1942. Div. 17-436.511-M3 

41. Development of Resistance Wire Strain Gauges {Final 
Report), Gaylord P. Harnwell, D3 284, U. Penn, Sep¬ 
tember 1942. 

Universit y of M innesota 
NDCrc-60 {42,43); OEMsr-75 {44,43) 

42. Progress Report Covering Thermistor Investigations, D3 18, 
U. Minnesota, Apr. 1, 1941. 


43. Final Report Covering Thermistor Investigations, Otto H. 
Schmitt, D3 52, U. Minnesota, June 1, 1941. 

Div. 17-451-MI 

44. Progress Report on Thermistor Investigation, Otto H. 
Schmitt, D3 143, U. Minnesota, Dec. 1, 1941. 

Div. 17-451-M6 

45. Final Report on Thermistor Investigation, Otto H. Schmitt, 
D3 177, U. Minnesota, Feb. 1, 1942. Div. 17-451-MI 

Yale University—N DCrc-107 

46. Report Number One—Contract NDCrc-107, [Fundamental 
Properties of Thermistors, and Their Applicability to 
Filters], Carol G. Montgomery, D3 27, Yale, Apr. 15, 1941. 

Div. 17-451-M2 

47. Report Number Two—Contract NDCrc-107, [Fundamental 
Properties of Thermistors, and Their Applicability to 
Filters], Carol G. Montgomery, D3 53, Yale, June 1, 1941. 

Div. 17-451-M2 

48. Final Report—Contract NDCrc-107, Carol G. Montgomery, 
D3 86, Yale, July 31, 1941. 

:fc ^ ♦ * 

49. Demonstration Set No. 1, D3 74. 

50. Summary Technical Report, Division 16, Volume 3, 
Chapter 8. 



OSRD APPOINTEES 


Miles J. Martin 
Francis L. Yost 

0. S. Duffendack 
Theodore Dunham, Jr. 
E. A. Eckhardt 
Harvey Fletcher 


Charles B. Bazzoni 


Division 17 


Chiefs 

George R. Harrison 
Paul E. Klopsteg 

Deputy Chiefs 

E. A. Eckhardt 
Paul E. Klopsteg 

Technical Aides 

Clark Goodman 

John A. Hornbeck (WOC) 

M embers 

William E. Forsythe 
George R. Harrison 
Herbert E. Ives 
Brian O’Brien 

Melville I. Stein 


Section 17.1 
Chief 

E. A. Eckhardt 

Deputy Chief 

Herbert E. Bragg 

Spec. Asst, to Chief 

John A. Hornbeck 

Technical Aide 

Herbert E. Bragg 

Members 

Semi J. Begun 

J. M. Cork 


212 


UWiiHillHB'l'J'i'ffL 


OSRD APPOINTEES ( Continued ) 


Gioacchino Failla 


Section 17.2 
Chief 

Melville I. Stein 

Technical Aide 
George E. Beggs 

Members 

C. H. Willis 


Section 17.3 
Chief 

Harvey Fletcher 

Spec. Asst, to Chief 

William S. Gorton 
L. J. Sivian 

Acting Chief 
P. M. Morse 

Technical Aides 

William S. Gorton (WOC) 

Members 

Davis Hallowell 
Floyd A. Firestone 

E. C. Wente 


J. C. Hubbard 


Clifford Morgan 

Vern 0. Knudsen 
Stanley S. Stevens 




213 



CONTRACT NUMBERS, CONTRACTORS, AND SUBJECTS OF CONTRACTS 


The contract information given below is for Division 17 work reported 
in (or related to) this volume. Contract information associated with 
Division 17 work reported in other volumes of the Division 17 Summary 
Technical Report is given in those volumes. 

The work under contracts whose numbers are marked with an 
asterisk (*) is not discussed anywhere in the Division 17 STR. For 
details of such work the reader is referred to the NDRC Bi-Monthly 
Summaries. The contracts themselves are listed here for completeness 
of contract information. 


Contract Number 

Name and Address of Contractor 

Subject 

NDCrc-22* 

Polytechnic Institute of Brooklyn 

New York, New York 

Preparation of pure amino guanidine sulphate. 

NDCrc-54 

Rensselaer Polytechnic Institute 

Troy, New York 

Studies and experimental investigations in connec¬ 
tion with the application of thermistors to elec¬ 
trical circuits of particular types. 

NDCrc-55 

Franklin Institute of the State of Pennsylvania 
Swarthmore, Pennsylvania 

Studies and experimental investigations in connec¬ 
tion with the application of thermistors to elec¬ 
trical circuits of particular types. 

NDCrc-60 

Regents of the University of Minnesota 
Minneapolis, Minnesota 

Studies and experimental investigations in connec¬ 
tion with the application of thermistors to elec¬ 
trical circuits of particular types. 

NDCrc-62 

Northwestern University 

Evanston, Illinois 

Studies and experimental investigations in connec¬ 
tion with the application of thermistors to elec¬ 
trical circuits of particular types. 

NDCrc-63 

National Cash Register/Company 

Dayton, Ohio 

Studies and experimental investigations in connec¬ 
tion with counter tubes. 

NDCrc-68 

University of Chicago 

Chicago, Illinois 

Studies and experimental investigations in connec¬ 
tion with counter tubes. 

NDCrc-84 

Carnegie Institute of Technology 

Pittsburgh, Pennsylvania 

Studies and experimental investigations in connec¬ 
tion with the application of thermistors to elec¬ 
trical circuits of particular types. 

NDCrc-102 

University of Pennsylvania 

Philadelphia, Pennsylvania 

Studies and experimental investigations in connec¬ 
tion with the present state of the use of strain 
gauges and their associated equipment in the 
testing of military vehicles with a comparison 
of existing methods, and a recommendation as 
to what is needed for the development of satis¬ 
factory telemetric strain gauges. 

NDCrc-140 

Western Electric Company, Inc. 

New York, New York 

Studies and experimental investigations in connec¬ 
tion with thermistor circuits. 

NDCrc-142 

Massachusetts Institute of Technology 
Cambridge, Massachusetts 

Studies and experimental investigations in connec¬ 
tion with the application of thermistors to elec¬ 
trical circuits of particular types. 

NDCrc-146 

Massachusetts Institute of Technology 
Cambridge, Massachusetts 

Studies and experimental investigations in connec¬ 
tion with counter tubes and circuits, to develop 
high-speed counter tube circuits in decade rings, 
covering the use of thyratrons, pentodes and 
double triodes, and the development of a “digi- 
tron” tube and counter circuit using this tube. 


214 


UyWfftiMMEWAL 








CONTRACT NUMBERS, CONTRACTORS, AND SUBJECTS OF CONTRACTS (« Continued ) 


Contract Number 


Name and Address of Contractor 


Subject 


NDCrc-179 


N DC re-189 


NDCrc-194 


OEMsr-110 
OEMsr-125 


OEMsr-155 


OEMsr-241 


OEMsr-247 


Massachusetts Institute of Technology 
Cambridge, Massachusetts 

The Trustees of the University of Pennsylvania 
Philadelphia, Pennsylvania 

Hazeltine Electronics Corporation 
New York, New York 


University of Pennsylvania 
Philadelphia, Pennsylvania 

University of Chicago 
Chicago, Illinois 

Massachusetts Institute of Technology 
Cambridge, Massachusetts 


The Board of Trustees of the University of Illinois 
Urbana, Illinois 


Rudolph Wurlitzer Company 
North Tonawanda, New York 


To conduct an investigation on the development 
of an instrument capable of measuring the radon 
content of breath and room air samples. 

To conduct an investigation on the development 
of a compact, portable and telemetering helium 
purity indicator. 

Studies and experimental investigations in connec¬ 
tion with telemetering readings, and, more par¬ 
ticularly, (i) conduct an investigation on the 
development of radio methods of telemetering 
readings of dials of airplane instruments neces¬ 
sary for blind flying, (ii) furnish, transportation 
paid, where and as directed by the Contracting 
Officer or an authorized representative, a work¬ 
ing model of at least one instrument capable of 
telemetering the airplane instrument dial read¬ 
ings, (iii) make modifications of the above de¬ 
scribed instrument as specified by the Con¬ 
tracting Officer or an authorized representative, 
(iv) conduct further studies and experimental 
investigations of the above subject as requested 
from time to time by the Contracting Officer, 
with particular reference to the requirements of 
various military and naval applications of the 
device, (v) deliver models of the modified instru¬ 
ment as requested by the Contracting Officer or 
an authorized representative from time to time, 
and (vi) conduct flight tests of the described 
instrument as directed by the Contracting Officer 
or an authorized representative. 

Studies and experimental investigations in connec¬ 
tion with the development of strain gauges. 

Studies and experimental investigations in connec¬ 
tion with an electronic ratchet tube for use as a 
high-speed counter. 

To study and investigate experimentally the ap¬ 
plication of strain gauges to the measurement of 
the expansion of gun barrels under hydrostatic 
pressure up to one hundred fifty thousand pounds 
(150,000 lb) per square inch, and to develop 
equipment for measurement of firing strains in 
field guns. 

Studies and experimental investigations in connec¬ 
tion with (i) the evaluation of the use of 3 to 20 
million volt x-rays for the radiography of thick 
metal sections, (ii) the construction of one (1) 
Kerst Electron Accelerator, together with ac¬ 
cessory equipment, (iii) the simplification of 
design of 4.5 million volt unit, and applications 
thereof, and (iv) the development and fabrication 
of ceramic bodies and glazes suitable for vacuum 
acceleration chambers for use with the 4.5 million 
volt and 20 million volt betatrons. 

Studies and experimental investigations in connec¬ 
tion with a special method of telemetering strain 
indications from an aircraft in flight, with means 
also available for the telemetering of aircraft 
instruments, in accordance with tentative speci¬ 
fications outlined by Section 17.2 of the National 
Defense Research Committee. 




215 










CONTRACT NUMBERS, CONTRACTORS, AND SUBJECTS OF 1 CONTRACTS ( Continued) 


Contract Number 

Name and Address of Contractor 

Subject 

OEMsr-254 

The Brush Development Company 

Cleveland, Ohio 

• 

Studies and experimental investigations in connec¬ 
tion with the development of (i) a transient 
strain analyzer involving a three-element oscil¬ 
lograph capable of recording high rates of de¬ 
formation in metals and of explosion pressures 
under water, (ii) an improved magnetic record¬ 
ing medium, and (iii) means for improvement 
of the recording mechanism with particular 
reference to the recording head. 

OEMsr-266 

Gulf Research and Development Company 
Pittsburgh, Pennsylvania 

% 

Studies and experimental investigations in connec¬ 
tion with the development of (i) improved 
methods of submarine mine control, (ii) a 
security device, (iii) an improved helium purity 
indicator for use in range-finders and lighter- 
than-air craft, (iv) a device for the determination 
of the quantity of fuel in the tanks of aircraft, 
(v) an indicator mine and associated devices and 
methods for determining the effectiveness of 
various explosive means of clearing minefields, 
and (vi) other instruments and devices of war¬ 
fare when and as requested in writing by the 
Contracting Officer or an authorized repre¬ 
sentative. 

OEMsr-274 

The National Cash Register Company 
Dayton, Ohio 

Studies and experimental investigations in connec¬ 
tion with (i) the development of improved elec¬ 
tronic tubes and circuits for use in high-speed 
counting devices, (ii) the development of a new 
system of secret communication involving elec¬ 
tronic counters, and (iii) the development of 
electronic and mechanical equipment. 

OEMsr-275 

The National Cash Register Company 

Dayton, Ohio 

Studies and experimental investigations looking 
toward the design and construction of special 
high-speed electric counter and recording units. 

OEMsr-278* 

Purdue Research Foundation 

Lafayette, Indiana 

To conduct studies and experimental investigation 
in connection with the development of improved 
radio control equipment for target airplanes, to 
perform preliminary tests of such equipment in 
a target airplane, and to furnish examples of the 
apparatus developed hereunder. 

OEMsr-294 

Massachusetts Institute of Technology 
Cambridge, Massachusetts 

(i) Design an x-ray generator capable of operating 
continuously at voltages up to two (2) million 
volts, under field conditions, with a high degree 
of reliability, (ii) construct five (5) such electro¬ 
static x-ray generators, capable of operating at 
voltages up to two (2) million volts or higher, 
(iii) train Navy personnel in the operation of 
such equipment, (iv) assist in installation of 
four (4) machines being supplied to the Navy 
Department, Bureau of Ordnance, and (v) stand¬ 
ardize design of all machines and supply stock 
of spare parts adaptable to all machines as a 
result of this standardization. 

OEMsr-314 

RCA Manufacturing Company, Incorporated 
Camden, New Jersey 

• 

Studies and experimental investigations in connec¬ 
tion with the development of a method of tele¬ 
metering aircraft instruments by means of tele¬ 
vision, including the development of a suitable 
television transmitter and receiver. 


216 


rnTiTnriiimfTTf 









CONTRACT NUMBERS, CONTRACTORS, AND SUBJECTS OF CONTRACTS ( Continued) 


Contract N umber 

Name and Address of Contractor 

Subject 

OEMsr-383* 

Swarthmore College 

Swarthmore, Pennsylvania 

Studies and experimental investigations in connec¬ 
tion with the development of a simple and cheap 
sound locator device for locating aircraft by 
civilian defense spotters. 

OEMsr-498 

Western Electric Company 

New York, New York 

Studies and experimental investigations in connec¬ 
tion with (i) obtaining an acoustical analysis of 
sounds from machine guns and from field pieces 
of larger caliber and (ii) obtaining an acoustical 
analysis of the sound from projectiles from these 
guns and of battle noises and other sounds of 
combat significance. 

OEMsr-600 

California Institute of Technology 

Pasadena, California 

Studies and experimental investigations in connec¬ 
tion with the development of (i) a scoring 
system which would give direct indication of the 
magnitude and direction of firing errors in shoot¬ 
ing anti-aircraft projectiles at a small towed 
glider, (ii) an acoustic device for the scoring 
of rockets fired from an airplane at a surface or 
airborne target, and (iii) an acoustic firing error 
indicator adapted for use with towed targets of 
several types used in anti-aircraft gunnery train¬ 
ing and calibrated for several calibers of fire; and 
the adaptation of the device to the scoring of 
rockets fired from planes at both aerial and 
surface targets. 

OEMsr-680* 

Massachusetts Institute of Technology 
Cambridge, Massachusetts 

Studies and experimental investigations in connec¬ 
tion with the development of ferro-electric salts 
whose curie temperatures occur at or near room 
temperature for use in microphones of increased 
sensitivity. 

OEMsr-823 

Hathaway Instrument Company 

Denver, Colorado 

Studies and experimental investigations in connec¬ 
tion with the development of multielement 
oscillograph and associated equipment according 
to specifications laid down by the Aberdeen 
Proving Ground, Aberdeen, Maryland. 

OEMsr-868 

Western Electric Company 

New York, New York 

(i) Studies and experimental investigations in con¬ 
nection with the development of a system in¬ 
cluding sound effects, records and loud speakers 
for reproduction of battle noises, and the furnish¬ 
ing of sound sequences simulating naval battles, 
and (ii) the instruction and training of Naval 
personnel in the operation and maintenance of 
the equipment developed. 

OEMsr-920 

Purdue Research Foundation, 

Purdue University 

Lafayette, Indiana 

Studies and experimental investigations in connec¬ 
tion with the development and construction of 
improved three-element cathode ray oscillo¬ 
graph, including three signal channel units and 
one timing unit. 

OEMsr-1037 

Princeton University 

Princeton, New Jersey 

Studies and experimental investigations in connec¬ 
tion with the (i) development of electronic 
commutation telemetering transmitting and re¬ 
ceiving units with associated amplifiers, input 
circuits, output circuits, and radio link, (ii) con¬ 
struction of one complete 18-channel commuta¬ 
tion telemetering system and flight test of this 
system, (iii) construction of two additional com¬ 
plete telemetering systems of at least eighteen 
(18) channels each, to serve as field test and 
manufacturing prototypes, (iv) development of a 


217 







CONTRACT NUMBERS, CONTRACTORS, AND SUBJECTS OF CONTRACTS (i Continued) 


Contract Number 


Name and Address of Contractor 


Subject 


OEMsr-1037 ( Continued ) cathode ray recording system to allow recording 

of the received commutation signal with simpli¬ 
fied apparatus adaptable to the Armstrong 
system of frequency modulated radio link, (v) de¬ 
velopment of an improved electronic commuta¬ 
tion system accommodating approximately thirty 
(30) items of intelligence, utilizing rotary beam 
scanning tubes for switching in place of present 
circuits, (vi) development of methods of sub- 
commutation for the transmission of additional 
items of intelligence in conjunction with the 
present system and the proposed new system, 
(vii) development and adaptation of a method 
of pulse modulation radio transmission to the 
transmission of signals derived from the rotary 
beam commutator, and (viii) development of 
telemetering system for LARK in accord with 
general specifications outlined in letter from 
Chief, BuAer to Chief, Research and Inventions, 
Navy Department, dated 19 July 1945, reference 
C21302, and construction, preliminary test, and 
delivery of one model of said system, including 
two complete ground receiving stations mounted 
in trucks to be provided by the Navy Department. 


OEMsr-1099 


OEMsr-1108 


OEMsr-1153 


OEMsr-1203 

testing of at least three (3) models of a device 
for indicating and recording the deflection time 
curve of a bulkhead or similar structure under 
the force of an explosion, and incorporate therein 
certain desirable bridge circuits which field tests 
have shown to be desirable, (ii) the development, 
construction, and testing of at least one model 
of a device for indicating and recording the 
deflection time curve of a small scale model of 
a bulkhead of similar structure under the force 
of an explosion, and (iii) the extension of operat¬ 
ing range of the device already developed under 
the subject contract in order to make it operable 
on deflections of the order of three (3) feet. 
Delivery of models and devices constructed 
hereunder together with six (6) exciter coils and 
three (3) spare receiving coils, detailed drawings, 
specifications, reports and directions for operation. 


C. G. Conn, Ltd. 
Elkhart, Indiana 


Mission Bell Radio Manufacturing Company, 
Incorporated 

(later Hoffman Radio Manufacturing 
Company) 

Los Angeles, California 

Allis-Chalmers Manufacturing Company 
Milwaukee, Wisconsin 


Faximile, Incorporated 
New York, New York 


Studies and experimental investigations in connec¬ 
tion with the development of fourteen (14) 
channel strain gauge telemetering equipment, in¬ 
corporating wattmeter circuits previously de¬ 
veloped. 

Studies and experimental investigations in connec¬ 
tion with the construction of thirty two (32) 
complete directional receivers for the firing error 
indicator and two thousand two hundred (2200) 
directional transmitters. 

Fabricate, assemble and deliver to the Govern¬ 
ment . . . Betatron equipment . . . manufactured 
in strict accordance with the written specifica¬ 
tions and instructions heretofore furnished to 
the vendor. 

Studies and experimental investigations in connec¬ 
tion with (i) the development, construction and 


218 


1 1 ii ii mil hi i'H'ii^ 







CONTRACT NUMBERS, CONTRACTORS, AND SUBJECTS OF CONTRACTS ( Continued) 


Contract Number 


Name and Address of Contractor 


Subject 


OEMsr-1211 


OEM sr-1241 


OEMsr-1308 


OEMsr-13G9 


OEMsr-1399 


OEMsr-1457 


White Research Associates 
Boston, Massachusetts 


Johns Hopkins University 
Baltimore, Maryland 


Shell Oil Company, Incorporated 
Houston, Texas 

The Texas Company 
New York, New York 


Raymond Rosen and Company 
Philadelphia, Pennsylvania 


Western Electric Company, Incorporated 
New York, New York 


Studies and experimental investigations in connec¬ 
tion with the (i) development and construction 
of four channel oscillograph equipment, including 
wide band direct coupled amplifiers, timing 
apparatus, and sweep circuits in accordance with 
preliminary specifications submitted by the 
Aberdeen Proving Ground; (ii) design and con¬ 
struction of high speed camera and developing 
apparatus to be used in connection with (i); 

(iii) installation of the equipment developed and 
constructed under (i) and (ii) hereof, in a trailer 
to be supplied by the Army Ordnance Depart¬ 
ment, as directed by the Scientific Officer and 
officials of the Aberdeen Proving Ground; and 

(iv) development and construction of one addi¬ 
tional trailer-mounted oscillograph unit in ac¬ 
cordance with Naval Proving Ground specifica¬ 
tions, and one dolly-mounted oscillograph unit 
for laboratory use. 

Studies and experimental investigations in connec¬ 
tion with (i) the development of non-destructive 
test apparatus for determining the seating of 
rotating bands on projectiles, with emphasis on 
apparatus which is simple, rugged, and rapid, 
to allow application to production testing, and 
(ii) associated methods for the determination of 
variations in sheet wall thickness. 

Studies and experimental investigations in connec¬ 
tion with the design and construction of a mobile 
timing laboratory. 

Studies and experimental investigations in connec¬ 
tion with the design and construction of one or 
more devices for the measurement of wall thick¬ 
ness of aircraft propeller blades, based upon the 
absorption, or other characteristics, of gamma 
radiation, or other suitable radiation. 

Manufacture of (in accordance with specifications 
and instructions) and delivery of two complete 
18 channel electronic strain gauge telemetering 
systems, including specified equipment. 

Studies and experimental investigations in connec¬ 
tion with the development for manufacture of a 
condenser microphone suitable for use in acoustic 
firing error indicator. 






219 











SERVICE PROJECT NUMBERS 


The projects listed below were transmitted to the Executive Secretary, 
NDRC, from the War or Navy Department through either the War 
Department Liaison Officer for NDRC or the Office of Research and 
Inventions (formerly the Coordinator of Research and Development), 
Navy Department. 


Service 


Project 


Number 

Subject 


Army Projects 


AC-20 

AC-34 


AC-40 

AC-41* 

AC-46 

AC-67 

AC-79 

AC-224.02 

AC-238.02 

OD-73 

OD-80* 

OD-90 

OD-102 

OD-124 

OD-140 

OD-148 

OD-151 

SC-27 

SC-111 

SC-134 


NA-133 
NA-134 
NA-152 
NO-123 
NO-123 
Ext. 
NO-173 
NO-260 
NS-197 


Electric strain gauge suitable for remote indicating and recording. 

Development of a thermistor and associated control circuits for use in heat responsive target seeking, controllable 
bombs. 

Development of telemetering equipment (later AC-224.02). 

Radio control of model aircraft. 

Firing error indicator. 

Design and development of apparatus for the air forces mobile instrument trailer. 

The development of some method or device for measuring the wall thickness of a hollow steel propeller blade. 
Research and development work on radio telemetering of strain gauge indications on aircraft (formerly AC-40). 
Research on magnetic recording materials (formerly SC-111). 

Multi-element oscillograph. 

Camera clock for range bombing instrumentation. 

Development of bomb instruments. 

Cathode-ray oscilloscope equipment. 

Studies of seismograph detectors for determining the time of impact of bombs. 

Mobile oscillograph. 

X-ray radiography—betatron. 

Methods of determining the seating of rotating bands on projectiles. 

Investigations of sound spectrum of ordnance equipment. 

Research on magnetic recording material (later 238.02). 

Rocket telemetering. 

Navy Projects 

Telemetering. 

Telemetering. 

Telemetering (14 or more channels indicating rapidly varying strain gauge resistances). 

Design, fabrication and installation of three two-million volt x-ray machines for mine or bomb recovery work. 

Design, research and development of sealed-off x-ray tube. 

Firing error indicator. 

Scoring of air to air rocket firing. 

Development of deflection-time measuring devices. 


*This project was assigned to Divisions 5 and 17. 


220 








INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. 
For access to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


AAF AC-67 instrument trailer, 128-134 
Absorption coefficients, high voltage 
X-rays, 140 

Accelerometer for determination of 
bomb retardation, 146-151 
calibration, 148-149 
Accumulator, high speed decimal, 168- 
170 

counting speed, 169 
resetting binary counter, 168-169 
Acoustic firing error indicator 

see Aperiodic FEI system; FEI, 
acoustic 

Acoustical tests, projectile banding, 178 
Air leakage test, projectile banding, 178 
Airborne pressure gauge, FEI, 71-72 
Aircraft flight testing, telemetering sys¬ 
tems 

see Telemetering systems 
Aircraft Radio Laboratory, Wright 
Field, 2, 20 

Allis-Chalmers betatron, 144 
Alnico powder, magnetic recording 
characteristics, 105 

Alpha particle activity indicator, 199- 

200 

Amplifier circuits 
OD-73 oscillograph, 111-112 
GD-102 oscillograph, 118-119 
OD-140 oscillograph, 125 
Putnam amplifier, 126 
Anthraquinone for helium purity indi¬ 
cator solution, 188 

Antiaircraft artillery requirements, 
FEI, 42-43 

Aperiodic FEI system, 58-64, 72-84, 
86-87 

design considerations, 58-59 
dial counters, 65, 81-82 
difference response lobes, 59-62 
electrographic tape recorder, 64 
errors, 61-62 
informing by lights, 82 
microphone, 72-75 
MOPA transmitter, 59 
noise elimination, 59-62 
quantitative response standardiza¬ 
tion, 82-84 

receiver, 63, 65, 75-81 
remedying defects, 61-62 
sum response zones, 59-60 
transmitter, 63, 73-75 
validation tests, 66-70 
Atomic bomb blast pressure measure¬ 
ments using FEI, 71 

Baldwin Southwark commutator tele¬ 
metering system, 16 
Ball rebound test, projectile banding, 
178 


Ballistic, measurements 
OD-140 oscillograph for ordnance 
engineering field measurements, 
119-128 

strain transient measurements on gun 
barrel surfaces, 198 
Ballistic shock waves, 45-52 

see also Aperiodic FEI system; FEI, 
acoustic 

absolute values of pressure ampli¬ 
tudes, 49-50 

amplitude dependence on range and 
projectile caliber, 51 
discontinuities, 47-49 
equipment for peak amplitude field 
studies, 54-55 

equipment for wave-form studies, 54 
photography of shock waves, 45, 
47-48 ' 

possible “after-waves”, 49 
relationship among amplitude, period, 
and miss-distance, 52 
shock wave cone, semi apex angle, 45 
shock wave period, 48 
sound spectrum, 192 
source of shock wave energy, 46 
velocity of propagation, 45, 47 
wave-form studies, 46-48, 86 
Banding machines, thermal testing 
method, 178 
Banding of projectiles 
see Projectile band tests 
Battle noise 

reproduction for training personnel, 
189-190 

sound spectrum, 191-192 
Beam tube, magnetically rotated, 27-28 
Bell Telephone Laboratories telemeter¬ 
ing systems, 27-29 
magnetically rotated beam tube, 
27-28 

multi-anode cathode ray tube, 28 
Berryllium copper tape for magnetic 
recording, 102 

Betatron for high voltage X-ray gener¬ 
ation, 135-136, 143-144 
advantages, 145 
energy range, 143 
voltage range, 138 

Biasing, a-c and d-c, in magnetic record¬ 
ing, 92-94 

Block I television system, 2 
Block III television system, 2, 10 
Boeing Aircraft Company telemetering 
system, 22, 29-30 

Bolometer elements, thermistors as, 194 
Bomb instrumentation, 146-154 

chronograph photographic tape, 147, 
150 

retardation measuring device, 146 151 
seismic detectors for measuring bomb 
impact, 151-154 


Brush Development Co. OS3B crystal 
galvanometer, 111-112 
Bulkhead deformation in underwater 
explosions, 155-161 
Bullets, magnetized, for FEI, 84 

California Institute of Technologv, 
39-87 

Camera, shock-wave triggered, 55-56 
Carnegie Institute of Technology, 194 
Cathode-ray oscillographs 

OD-73 multi-element type, 108-113 
GD-102 high-speed drum camera 
type, 113-119 

OD-140 mobile multi-channel type, 
119-128 

Cathode-ray tube, multi-anode type, 28 
Cellulose acetate tape as magnetic 
recording carrier, 88-105 
C. G. Conn telemetering system, 17-18 
Click remover circuit, MTR, 95-97 
Cobalt-nickel alloys for magnetic re¬ 
cording, 88 

Coercive force, magnetic recording 
theory, 90-91 

Coercive force/remanence ratio, 105- 
106 

of evaporated films, 105 
Color-change indication of helium 
purity, 187-188 

Commutation telemetering systems, 
3-4, 9-14, 23-31 
advantages, 4, 9 

Baldwin Southwark system, 16-17 
bridge-voltage supply, 14 
BTL system, 27-29 
direct commutation, 10 
flight test results, 30-31 
frequency response, 12-13 
Julien Friez system, 23 
linearity requirements, 14 
magnetically rotated beam tube, 27- 
28 

multi-anode cathode ray tube, 28 
NAES system, 16-17 
Princeton University systems, 23-27 
principle of commutation, 9 
reduction of cross talk, 12-13 
sampling speed, 11 
signal generator, 14 
subcommutation system, 11 
synchronization, 14 
television, 10 
Vultee radio recorder, 23 
Compression tests for projectile bands, 
179, 181-182 

Compton effect, high voltage X-rays, 
138-140 

Consolidated Vultee Aircraft Corpora¬ 
tion telemetering system, 23 
Controlled impulse generator, 170 



221 


222 


INDEX 


Counter circuits, electronic 
see Electronic counter circuits 
Current and potential contact tests of 
projectile bands, 178 
Curtiss-Wright telemetering system, 
14-16 

Decimal counters, electronic, 168-176 
high-speed decimal accumulator, 168- 
170 

Definition, high voltage X-ray radio¬ 
graphs, 140 

Deflection-time measuring devices, 155- 
161 

electromagnetic deflection unit, 155- 
158 

optical deflection gauge, 155,158-161 
Dekatron counter circuit, 174 
Delay error, FEI, 55-57, 61 
Demagnetization, magnetic recording, 
90-91 

Difference response lobes, aperiodic 
FEI system, 59-62 

Doppler effect errors, aperiodic FEI 
system, 61 

ECD (electronic counter communica¬ 
tion device), 170-172 
EG (electronic gate), 168, 170, 172 
Electrographic tape recorder, aperiodic 
FEI system, 64 

Electromagnet ic deflection unit (EMU), 
155-158 

Electron ratchet tube, 173-174 
Electronic counter circuits, 167-176 
application to muzzle velocity 
measurements, 168 
binary counter, 167, 169, 175 
binary to decimal converter, 175 
conjugate-pair counter circuit, 173- 
174 

controlled impulse generator, 170 
counting speed, 167, 168 
decimal counters, 168-176 
dekatron counter circuit, 174 
electron ratchet tube, 173-174 
electronic counter communication 
device, 170-172 
gate circuit, 168, 170, 172 
high speed decimal accumulator, 
168-170 

thyratron counters, 174, 175 
Electronic gate circuit, 168, 170, 172 
Electroplated ribbon (ER), magnetic 
recording, 88, 94, 99-103 
Electrostatic generator, high voltage 
X-rays, 135-138, 143-144 
EMU-3B range expander unit, 158 
Erasing, magnetic recording, 91-94 
Evaporation method, magnetic record¬ 
ing media studies, 100-102, 104- 
105 

Exposure time, high voltage X-ray 
radiographs, 139-141 

Faximile, Inc., 156 

Fe 3 0 4 , synthetic, for magnetic recording 
tape, 88, 105 


FEC (firing error camera), 55-56, 66-69 
Fe-Co alloys for magnetic recording, 
104 

FEI, acoustic, 38-87 

see also Aperiodic FEI system 
aerial gunnery requirements, 43-44 
antiaircraft artillery requirements, 

42- 43 

application to atomic bomb blast 
pressure measurement, 71-72 
application to rocket fire scoring, 70 
courses followed by towing plane, 

43- 44 

delay error, 57, 61 
directionality of firing error, 41 
dual microphone system, 57 
general design considerations, 40-41, 
57-59 

gun target line, definition, 52 
harmonic mean miss distance, 67, 68 
informing and scoring functions, 41- 
42 

limitations, 38, 69-71 
maximum rate of fire, 43 
Navy requirements, 44 
reproducibility of response, 51, 62 
resonant FEI, 58-59, 85 
response patterns, 52 
scoring accuracy, 39, 68-69 
“static firing” field tests, 52-54 
theory, 38 

validation of results, 39, 55-56, 66-70 
FEI, magnetic method, 84 
Filter type subcarrier telemetering sys¬ 
tems, 5, 21 

Firing error camera (FEC), 55-56, 
66-69 

Firing error indicator, acoustic 
see FEI, acoustic 

Firing error oscillograph (FEO), 55 
Flight testing of aircraft, telemetering 
systems 

see Telemetering systems 
Flowmeters, telemetric 
Pirani-gauge, 195 
rotor-type, 195-196 
thermistor-type, 195 
F-m subcarrier telemetering system, 6, 
21 

Franklin Institute, 195 
Frequency response of telemetering 
systems 

commutation systems, 9-10, 12-13 
requirements, 1, 4, 12-13 
telvision systems, 10 
Friez and Sons telemetering system, 23 
Fuel consumption measurement by 
telemetric flowmeters, 195-196 


Galvanometers 

in AAF instrument trailer oscillo¬ 
graph, 128-129 
in OD73 oscillograph, 108-111 
Gamma-ray transmission for measuring 
steel wall thickness, 162-166 
Gap width, magnetic recording, 93-94 
effective gap width, definition, 93 
for ring heads, 98 


AL 


Gate circuits in electronic counters, 
168, 170, 172 
Geiger-Mueller counter 

for measuring propeller wall thick¬ 
ness, 162-165 

for testing projectile bands, 179- 

181 

Geophones for seismographic ranging, 
132 

Guided missiles, telemetering systems, 
1, 31 

Gulf Research and Development Com¬ 
pany, 146, 187, 195 
Gunnery training 

see Aperiodic FEI system; FEI, acous¬ 
tic 

Harmonic mean miss-distance, FEI 
figure-of-merit, 67-68 
Harvard University, 195 
Hathaway Instrument Company oscil¬ 
lograph research, 108-113 
Hazeltine Service Corporation tele¬ 
metering system, 2 

Heat flow method, projectile band tests, 
178 

Helium-purity indicators, 183-188 
color-change indicator, 187-188 
indicator solution, 188 
velocity-of-sound indicator, 183-187 
Herzog test for projectile banding, 178 
Heterodyne subcarrier telemetering 
systems 

Curtiss-Wright system, 14-16 
general design considerations, 5 
High-speed decimal accumulator, 168- 
170 

counting speed, 169 
resetting binary counter, 168-169 
High-voltage X-ray radiography 

see X-ray radiography, high-voltage 
HK-257B, Hytron-type tube for oscil¬ 
lograph amplifier, 118 
HPI (helium purity indicators), 183- 
188 

color change indicators, 187-188 
indicator solution, 188 
velocity-of-sound indicator, 183-187 
HSDA (high speed decimal accumu¬ 
lator), 168-170 
counting speed, 169 
resetting binary counter, 168-169 
Hytron-type tube used in oscillograph 
amplifier, 118 

Inductance method for testing bands 
on 105 mm shells, 178 
Instrument trailer, AAF AC-67; 128 - 
134 

galvanometer, 128-131 
military requirements, 128-129 
timing device, 131 

Inyo Kern Naval Ordnance Testing 
Station, 44 

Iron-cobalt alloys for magnetic record¬ 
ing, 99, 104 

Klipsch test for projectile bands, 179, 
181-182 



INDEX 


223 


Latitude (thickness) in high voltage 
X-ray radiographs, 140-142 
Light unit, ODG, 158-161 
Linearity requirements of telemetering 
radio systems, 3-4, 7-8, 14 
Link Co. f-m transmitter and receiver, 
15, 16, 22, 27 

Loudspeaking system for battle noise 
reproduction, 189-190 
frequency response, 189 
listening range, 190 

Machlett Laboratories, X-ray tube, 144 
Magnetic recording, 88-107 

a-c biasing and obliteration, 92-94 
advantages, 88-89 
coercive force/remanence ratio as 
figure of merit, 90-91 
d-c obliteration, 92-93 
demagnetization, 90-91 
frequency dependence of output volt¬ 
age, 88-92 

gap width of recording head, 93-94 
low frequency AM carrier frequency 
system, 92 

magnetic transient recorder, 95-97 

number of reproductions possible, 92 

penetration effect, 91-92 

phase distortion, 92 

remanent induction, 90-92 

ring head, 88, 98-99 

theory, 88-92 

Magnetic recording media, 97-106 
beryllium copper tape, 99, 102 
cobalt-nickel alloys, 88 
electroplated ribbon, 88, 94, 97, 
100-103 

electroplating solution, 99 
evaporation studies, 100-102, 104- 
105 

iron cobalt alloys, 99 
military requirements, 97-98 
paper tape, 88 
plastic tape, 88, 105 
powder coated tapes, 88, 100 102, 
105-106 

steel tape, 92, 95, 102 
Magnetic reluctance strain gauges, 196 
Magnetic transient recorder, 88, 92-97 
click remover circuit, 95-97 
frequency response range, 92, 95 
military requirements, 92 
phase shift elimination, 95 
Magnetically rotated beam tube, 27-28 
Magnetite, synthetic, for magnetic re¬ 
cording tape, 88, 105-106 
Magnetizable tapes for magnetic re¬ 
cording 

see Magnetic recording media 
Magnetized bullets for FEI, 84 
Manganese steel tape for magnetic 
recording, 102 

Marksmanship scoring device 
see FEI, acoustic 

Martensitic steel, magnetic recording 
characteristics, 105 

Massachusetts Institute of Technology, 
135, 168, 194, 199 


Micrometer, electric, for gun barrel 
measurements, 198-199 
Microphones 
aperiodic FEI, 72-75 
for seismographic ranging, 132 
piezoelectric quartz crystal sound 
cell, 54 

Miss-distance, FEI 
as measure of pressure amplitude, 49 
harmonic mean miss-distance, 67 
in terms of shock wave period, 48-49 
measured by FEC, 55-56, 66 
measured by SPT, 68 
miss-distance zones, 42-44, 67 
MOPA transmitter, FEI, 59, 73 
MTR (magnetic transient recorder), 
88, 92-97 

click remover circuit, 95-97 
frequency response range, 92, 95 
military requirements, 92 
phase shift elimination, 95 
Multi-anode cathode ray tube, 28 
Mu-metal strips in MTR recording 
head, 95 
Muzzle velocity 

effect of powder irregularities, 178, 

181 

effect of projectile banding clearance, 
177-178, 181-182 

measurement using electronic coun¬ 
ter, 168 

Muzzle waves, sound spectrum, 191-192 

National Broadcasting Company tele¬ 
metering system, 2 

National Cash Register Company, 168 
Naval Aircraft Experimental Station 
telemetering system, 16-17 
Negative resistance of thermistors, 193 
Noise discrimination, aperiodic FEI 
system, 59, 61-62 

Noise reproduction for battle personnel 
training, 189-190 
Northwestern University, 195 

Obliteration, magnetic recording, 91-94 
OD-73 multi-element oscillograph, 108- 
113 

amplifier, 111-112 
frequency range, 111 
galvanometers, 108-112 
military requirements, 108-111 
optical housing, 111 
recording mechanism, 111 
temperature compensation, 110, 111 
OD-102 high speed cathode-ray drum 
camera oscillograph, 113-124 
assembly, 116-124 
basic circuits, 118-119 
military requirements, 113-116 
OD-140 mobile multi-channel cathode- 
ray oscillograph, 119-125 
amplifier, 119-122, 125, 126 
camera, 119-122, 125, 128 
timing pulse circuits, 127 
timing unit, 119-123, 126 
voltage calibration, 119-123 
ODG (optical deflection gauge) 



S 2 e Optical deflection gauge 
105-mm shells, banding tests, 177-182 
Optical deflection gauge (ODG), 155, 
158-161 

light unit, 158-160 
power unit, 158, 159 
Optical lever strain gauges (ODG), 196 
Ordnance equipment noise, sound spec¬ 
trum, 191-192 
Orthicon camera tube, 2 
OS3B crystal galvanometer, 111-112 
Oscillator, RC-tuned, used in VS I 
183-185 

Oscillographs, 108-134 

AAF AC-67 instrument trailer, 128- 
134 

OD-73 multi-element oscillograph, 
108-113 

OD-102 high-speed drum camera 
cathode ray oscillograph, 113— 

119 

OD-140 mobile multi-channel cath¬ 
ode-ray oscillograph, 119-128 

Paper tape as magnetic recording 
carrier, 88 

Penetration effect, magnetic recording 
theory, 91-92 

Penetron for measuring wall thickness 
of a pipe, 162 

Personnel training, battle noise repro¬ 
duction, 189-190 

Phase distortion, magnetic recording, 92 
Phase modulated subcarrier telemeter¬ 
ing system, 21 

Phase shift elimination, MTR, 95 
Phonic wheel generator for Conn tele¬ 
metering system, 17 
Phosphor bronze ribbon for magnetic 
recording, 88, 102 

Photoelectric effect, high voltage X- 
rays, 138-140 

Pioneer autosyn-magnasyn amplifier, 18 
Pirani-gauge flowmeter, 195 
Planck’s law, 139 

Plastic tape as magnetic recording 
carrier, 88, 105 

Potter-Bucky diaphragm, 142-143 
Poulsen, Valdemar, telegraphone, 88 
Powder coated tape (PCT), 88, 100- 
101, 105-106 

cellulose acetate tape, 88, 105 
magnetite powder, 88, 105-106 
martensitic steel powder, 105 
method of application, 105 
paper tape, 88 
Power unit, ODG, 158-159 
Princeton University 

telemetering systems, 3, 23-27 
thermistors for submarine mines, 195 
Projectile band tests, 177-182 
acoustical tests, 178 
ball rebound test, 178 
compression tests, 179, 181-182 
current and potential contact tests, 
178 

Herzog test, 178 
inductance method, 178 



224 


INDEX 


Klipsch method, 179, 181-182 
supersonic tests, 178 
thermal tests, 178 

use of Geiger-Mueller counter, 179- 
181 

X-ray methods, 179-181 
Propeller blades, wall thickness meas¬ 
urements, 162-166 

Pulse-modulation subcarrier telemeter¬ 
ing system, 18-21 

Purdue University oscillograph re¬ 
search, 113-119 
Putnam amplifier, 126 

Radio link, telemetering systems 
f-m transmitter and receiver, 15, 16, 
22, 27 

frequency response requirements, 3- 
4, 12-13, 26 

linearity requirements, 3-4, 7, 14 
modulation limiter, 8-9 
Radiosonde transmitting equipment, 23 
Radioactive selenium used in propeller 
wall thickness measurements, 162 
Radiography, high voltage X-ray 
see X-ray radiography, high voltage 
Radium poisoning, radon indicator, 
199-200 

Range bombing, seismic detection of 
time and position of impact, 
151-154 

Range expander unit (REU), 158 
Rankine-Hugoniot discontinuity rela¬ 
tionship, 48, 50 

RC tuned oscillator used in VSI, 183- 
185 

Reactance gauges, 4, 6 
Receiver, aperiodic FEI, 63, 65, 75-81 
automatic frequency control, 76 
filter properties, 75-77 
modification for atomic bomb blast 
pressure measurements, 71 
operational analysis, 75-80 
physical characteristics, 80-81 
pulse lengthening of signals, 75-79 
spurious shot records, 75-77 
standardization, 83 

Recommendations for future research 
FEI system, 69 

television telemetering systems, 10 
thermistor design and applications, 
194 

Reconnaisance procedure, seismographic 
ranging, 152 

Recording, magnetic tape 
see Magnetic recording 
Remanent induction, magnetic record¬ 
ing theory, 90-92 

Rensselaer Polytechnic Institute, 194 
Resetting binary counter, 169 
Resistance wire strain gauges, 196-198 
specifications, 196 

temperature compensation, 196, 197 
Resonant FEI, 58-59 
REU (Range expander unit), 158 
Ring head (RH) for magnetic recording 
and reproducing, 88, 98-99 
Rockets for plane to plane shooting 


FEI application, 44 
launching arrangement, 44 
Rotary beam tube, 27-28 

Scattering of X-rays, effect on radio¬ 
graphs, 139, 142-143 
Secondary radiation, effect on X-ray 
radiographs, 139, 142-143 
Seismic detectors, bomb impact meas¬ 
urements, 151-154 
Seismographic ranging 

reconnaisance and calibration pro¬ 
cedure, 152 

use of microphones and geophones, 
132 

Selenium, radioactive, for measuring 
propeller wall thickness, 162 
Sensitivity, high voltage X-ray radio¬ 
graphs, 140, 142-143 
effect of scattered radiation, 142-143 
Potter-Bucky diaphragm for reduc¬ 
ing scattering effect, 142-143 
Shell Oil Company oscillograph re¬ 
search, 128-134 
Shock waves, ballistic 
see Ballistic shock waves 
Sodium hydrosulfite for helium purity 
indicator solution, 188 
Sodium hydroxide for helium purity 
indicator solution, 188 
Sound recording 

battle noise recordings for personnel 
training, 189-190 
magnetic recording, 88-107 
Sound spectrum of ordnance equipment 
and battle noises, 191-192 
ballistic shock waves, 192 
muzzle waves, 191-192 
recording system, 191 
Spectral distribution, high voltage X- 
rays, 139 

SPT (Stibitz dual photographic theodo¬ 
lite), 66-69 
Static firing, FEI 
field tests, 52-54 
frequency-shift standardization, 83 
Steel tape for magnetic recording, 95 
Stibitz dual photographic theodolite, 
66-69 

Strain gauge telemetering systems 
see Telemetering systems 
Strain gauges, 196-198 

magnetic reluctance type, 196 
measurement of strain transients on 
gun barrel surfaces, 198-199 
optical lever type, 196 
resistance wire type, 196-198 
temperature compensation, 196 
Strain transient measurements on gun 
barrel surfaces, 198-199 
Subcarrier telemetering systems, 3-10, 
14-21 

amplitude modulated, 21 
Boeing system, 22 
bridge balancing, 9 
C. G. Conn system, 17-18 
cross-modulation frequencies, 7-8 
Curtiss-Wright system, 14-16 


filter type, 5, 21 
frequency modulated, 6, 21 
frequency stability of subcarriers, 9 
heterodyne subcarrier system, 5-6, 
14-16 

linearity requirements, 7-9 
modulation limiter, 8 
NAES system, 16-17 
ordinary subcarrier, 5-16 
phase discrimination, 9 
phase-modulated, 21 
pulse-modulation system, 18-21 
wattmeter frequency selector, 6, 17 
Wurlitzer systems, 18-22 
Subcommutation telemetering system, 
11 

Sum response, FEI, 40, 59 
Supersonic tests, projectile banding, 
178 

Telemetering, 1-37 
circuits, 32-37 

combination of subcarrier and com¬ 
mutation systems, 14, 29 
comparison of subcarrier and com¬ 
mutation systems, 3-4 
firing error indicator, 57-66 
guided missile telemetering, 31 
historv of research and development, 
2-4 

military requirements, 1-2 
noise factors, 4 
test results, 30 

thermistor flow meter, 194-196 
time sequence recorder, 128-134 
types of systems, 3-4 
Telemetering systems, commutation 
BTL system, 27-29 
Consolidated Vultee system, 23 
direct commutation, 10 
frequency response of radio link, 
12-i3 

Friez system, 23 
linearity of radio link, 14 
Princeton University system, 23-27 
signal generator, 14 
subcommutation, 11 
synchronization, 14 
television, 10 

Telemetering systems, subcarrier 
Boeing system, 22 
C. G. Conn system, 17-18 
Curtiss-Wright system, 15, 16 
frequency modulated, 6, 21 
frequency stability, 9 
heterodyne subcarrier, 5-6, 14-16 
linearity of radio link, 7 
modulation of radio link, 8-9 
NAES system, 16-17 
ordinary subcarrier, 5, 16 
phase discrimination, 9 
wattmeter principle, 6 
Wurlitzer systems, 18-21 
Telemetric flowmeters, 195-196 
Television telemetering systems 
Block I, 2 

Curtiss-Wright system, 14-16 
evaluation, 10 



INDEX 


225 


frequency response, 10 
NAES system, 16-17 
NBC Block III system, 2, 10 
recommendations, 10 
Temperature-compensated strain gauge, 
196-198 

Texas Company, 162-166 
Thermal test of projectile banding, 178 
Thermistors, 193-195 
applications, 193-195 
as bolometer elements, 194 
negative resistance, 193 
Thyratron counter circuits, 174-176 
Time sequence recorder, 128-134 
Time-division multiplex telemetering 
systems 

see Commutation telemetering sys¬ 
tems 

Training devices 

battle noise reproduction, 189-190 
firing error indicator, 39-87 
Transmitter, aperiodic FEI, 62, 73-74 
simplification for atomic bomb blast 
pressure measurements, 71 
standardization, 82-83 
Tungsten-steel alloy for magnetic re¬ 
cording, tape, 92 

Underwater explosion damage, deflec¬ 


tion-time measuring devices, 
155-161 

University of Chicago, 168 
University of Illinois, 135 
University of Minnesota, 194 
University of Pennsylvania, 183 

Van de Graaff electrostatic generator, 
for high voltage X-rays, 135-138, 
143-144 

Velocity-of-sound helium purity indi¬ 
cator, 183-187 
calibration, 185 
RC oscillator, 183-185 
Vultee radio recorder, 23 

Wall thickness of hollow steel pro¬ 
peller blades, measuring device, 
162-166 

Wattmeter frequency selector subcar¬ 
rier telemetering system, 6, 17 
Western Electric Company, 189, 191 
White Research Associates, oscillo¬ 
graph research, 119-128 
Wire recording 

see Magnetic recording 
Wurlitzer pulse-modulation telemeter¬ 
ing system, 18-21 


X-ray radiography, high voltage, 135- 
145 

absorption coefficient, 140 
advantages of high voltage, 140-143 
betatron, 135-136, 143-145 
Compton effect, 138, 139 
definition, 140-142 
effect of scattered radiation, 139-140, 
142-143 

exponential absorption equation, 140 
exposure time, 139-141 
latitude, 140-142 

maximum X-ray energy formula, 
139 

optimum tube voltage, 139 
pair production, 138-139 
photoelectric effect, 138, 139 
Planck’s quantum hypothesis, 139 
Potter-Bucky diaphragm, 142 
sensitivity, HO, 142-143 
spectral energy distribution, 139 
spot size, 141-142 
upper voltage limit, 139 
Van de Graaff electrostatic gener¬ 
ator, 135-138, 144 

X-ray tests of projectile banding, 179- 
181 

Yale University, 194 
































































































































































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